In a word, no when it comes to the general extent, as well as to most individual cases (currently). But there are nuances.
It would be useful to look at some of the denier tactics to see how the imperfections of historiography are abused by them.
First of all, let's define our terms. By Holocaust I mean what institutions like Yad Vashem and USHMM mean by it, namely the extermination specifically of Jews.
If one uses an unofficial extended definition to mean all the victims of the Nazi genocide, this complicates the question somewhat: many people wrongly believe in the "11 million" myth that irks historians (see https://www.timesofisrael.com/remember-the-11-million-why-an-inflated-victims-tally-irks-holocaust-historians/ on that). On the other hand the numbers of non-Jewish non-combatants killed by the Nazis do range from 7 to 8 million according to this survey: http://holocaustcontroversies.blogspot.com/2008/04/5-million-non-jewish-victims-part-2.html
Is someone using the wrong figure diminishing the extent? Maybe, if only inadvertently. But let's go back to the deliberate denial and minimization.
Let's take the Auschwitz death toll. Up until the beginning of the 1990s there was no scholarly consensus on it. The East Bloc publications usually used the number 4 million (not referring to Jews but to all victims) or the range 2.5-4m. The Western publications were all over the place, with some accepting the commandant Rudolf Hoess' earlier figure of 2.5-3 million Jews (but ignoring his later testimony that this figure was nonsense and the real one is closer to 1 million), some using the transport data (like Raul Hilberg, who estimated 1m Jewish victims), some used weird mixed estimates, like Deborah Lipstadt, who in her book Beyond Belief posited that at least 2 million Jews and about 2 million non-Jews died in Auschwitz (though in her defense, she's not, strictly speaking, a Holocaust historian).
With such a smorgasbord of opinions it's not hard to see how deniers would seize on the issue of the exact numbers to deny the whole. Even though an honest study of the basic sources at that time would still show that Auschwitz was an extermination camp where at least hundreds of thousands were killed, even if the exact number would be hard to establish without a deeper study.
Such a study was performed by the Auschwitz museum historian F. Piper, who analyzed the balance of the incoming and outgoing transports as well as the deaths of registered prisoners to reach the difference of about 1.1 m deaths, of them about 1m Jews. While this exact number is not set in stone (the latest developments regarding the so-called transit Jews, whose number Piper underestimated by about 80k, indicate that the Jewish death toll is likely more close to 900k and the overall death toll to 1m - albeit the mainstream scholarship has not yet caught up with this), the magnitude pretty much is. So someone e.g. halving the death toll in this day has not done their homework.
Another example would be the Majdanek death toll. Right after the liberation of the camp by the Soviets figures like 1.5 and 1.7 million were thrown around (again, not referring to Jews). But they were soon revised down to 360k even in the Communist Poland itself. Then to about 230k. And recently to 78k, of whom about 59k were Jews. Notably, it seems that the non-Jewish part shrunk (the Jewish one was estimated by Hilberg as 50k already in 1961).
(This is the case where the pre-revision estimate of the Majdanek death toll as 40000 by two leading deniers was ironically closer to the reality than the then current official museum estimate.)
Similar revisions, albeit usually not as drastic as in the case of Majdanek, have happened in practically every camp. The usual scenario was: the exaggerated "fog of war" figure is repeated in the 1945+ press based on some crude estimates and assumptions. Some time later scholars (or official investigators) begin to analyze the actual documents, perhaps in a more calm atmosphere, and the figure goes down.
At this point one might empathize with a layman who might ask: "how do I know the current figures are correct if so many of the accepted old ones turned out to be exaggerations". I don't have a solution except to study the individual estimates yourself. As someone who has done this, I know that the current figures for the most important camps are "stable" in that they won't change their order of magnitude and most are unlikely to change by more than 10% either way. Another point: simply dismiss the immediate post-liberation figures from newspapers and propaganda reels. They were never scholarly so they're not a part of the picture we're discussing.
Also remember that knowledge always progresses; a century ago the physics looked very different from what we know today, yet it was still science. So it is with history. In fact, the process of the numbers revision has not ended. Just relatively recently (2001) an important document - the so-called Höfle telegram from the British decodes - was published which helped to make the estimates for Treblinka, Belzec and Sobibor a lot more precise. It's normal. And the scholars are willing to revise based on evidence, so the denier accusations of "dogmatism" etc. are obviously nonsensical.
So one might ask: how does this revision of the individual numbers affect the overall Holocaust death toll?
The somewhat surprising answer is: not much. See, the overall death toll was never estimated by simply summing up individual sums for camps or Einsatzgruppen shootings. That was never an option anyway given all the unknowns - e.g. all the Jews who died working in the thousands of smaller slave labor camps, the victims of the numerous smaller shootings (especially as not only Einsatzgruppen were shooting, but numerous other mobile units), victims of the ghetto hunger, etc.
Rather, the overall death toll is and can only be based on the demographic studies, such as this one: https://www.degruyter.com/viewbooktoc/product/220100
One really should go over all the individual regions and gather as much information as possible on the pre-war numbers, post-war numbers, deportation stats, stats on those returned etc. Given the incomplete data for some regions the overall death toll will always vary from scholar to scholar, and even individual scholars will usually offer an uncertainty range for their estimates.
The number "six million" is nothing more than a media shortcut. I suppose it cannot be expected that each time the Holocaust death toll is mentioned, it would be formulated like "most scholarly death tolls are between 5 and 6 million". So people usually go with the upper range.
But you see that in this uncertainty of 1 million there's also some built-in buffer for the uncertainties of the individual camps or killing actions. If e.g. in some camp the estimate goes down by 100k, it may be that so many people died somewhere else; or maybe that the overall death toll is that closer to 5 million; or a combination of both.
What it doesn't mean is that "only 250k died everywhere" as so many denier zombies like to claim. Going from scholarly uncertainty to outright denial is always a quantum leap of logic.
Tl;dr: estimates for individual camp death tolls or killing action tolls often vary with time; the estimates one can see in the immediate postwar years are more often exaggerated than not and should not be taken seriously; nowadays we know more, not less, so today's estimates are more precise and stable; even today the process of revising the individual estimates is ongoing, albeit the variation is significantly lower; the overall death toll estimates usually fall between 5 and 6 million and do not directly depend on the sums of the individual camp estimates, rather they're based on the demographic data; deniers dishonestly abuse the normal scholarly uncertainties to deny or minimize the whole.
Hi! As this question pertains to basic, underlying facts of the Holocaust, I hope you can appreciate that it can be a fraught subject to deal with. While we want people to get the answers they are looking for, we also remain very conscious that threads of this nature can attract the very wrong kind of response. As such, this message is not intended to provide you with all of the answers, but simply to address some of the basic facts, as well as Holocaust Denial, and provide a short list of introductory reading. There is always more than can be said, but we hope this is a good starting point for you.
##What Was the Holocaust?
The Holocaust refers the genocidal deaths of 5-6 million European Jews carried out systematically by Nazi Germany as part of targeted policies of persecution and extermination during World War II. Some historians will also include the deaths of the Roma, Communists, Mentally Disabled, and other groups targeted by Nazi policies, which brings the total number of deaths to 11-17 million. Debates about whether or not the Holocaust includes these deaths or not is a matter of definitions, but in no way a reflection on dispute that they occurred.
##But This Guy Says Otherwise!
Unfortunately, there is a small, but at times vocal, minority of persons who fall into the category of Holocaust Denial, attempting to minimize the deaths by orders of magnitude, impugn well proven facts, or even claim that the Holocaust is entirely a fabrication and never happened. Although they often self-style themselves as "Revisionists", they are not correctly described by the title. While revisionism is not inherently a dirty word, actual revision, to quote Michael Shermer, "entails refinement of detailed knowledge about events, rarely complete denial of the events themselves, and certainly not denial of the cumulation of events known as the Holocaust."
It is absolutely true that were you to read a book written in 1950 or so, you would find information which any decent scholar today might reject, and that is the result of good revisionism. But these changes, which even can be quite large, such as the reassessment of deaths at Auschwitz from ~4 million to ~1 million, are done within the bounds of respected, academic study, and reflect decades of work that builds upon the work of previous scholars, and certainly does not willfully disregard documented evidence and recollections. There are still plenty of questions within Holocaust Studies that are debated by scholars, and there may still be more out there for us to discover, and revise, but when it comes to the basic facts, there is simply no valid argument against them.
##So What Are the Basics?
Beginning with their rise to power in the 1930s, the Nazi Party, headed by Adolf Hitler, implemented a series of anti-Jewish policies within Germany, marginalizing Jews within society more and more, stripping them of their wealth, livelihoods, and their dignity. With the invasion of Poland in 1939, the number of Jews under Nazi control reached into the millions, and this number would again increase with the invasion of the Soviet Union in 1941. Shortly after the invasion of Poland, the Germans started to confine the Jewish population into squalid ghettos. After several plans on how to rid Europe of the Jews that all proved unfeasible, by the time of the invasion of the Soviet Union in 1941, ideological (Antisemitism) and pragmatic (Resources) considerations lead to mass-killings becoming the only viable option in the minds of the Nazi leadership. First only practiced in the USSR, it was influential groups such as the SS and the administration of the General Government that pushed to expand the killing operations to all of Europe and sometime at the end of 1941 met with Hitler’s approval.
The early killings were carried out foremost by the Einsatzgruppen, paramilitary groups organized under the aegis of the SS and tasked with carrying out the mass killings of Jews, Communists, and other 'undesirable elements' in the wake of the German military's advance. In what is often termed the 'Holocaust by Bullet', the Einsatzgruppen, with the assistance of the Wehrmacht, the SD, the Security Police, as well as local collaborators, would kill roughly two million persons, over half of them Jews. Most killings were carried out with mass shootings, but other methods such as gas vans - intended to spare the killers the trauma of shooting so many persons day after day - were utilized too.
By early 1942, the "Final Solution" to the so-called "Jewish Question" was essentially finalized at the Wannsee Conference under the direction of Reinhard Heydrich, where the plan to eliminate the Jewish population of Europe using a series of extermination camps set up in occupied Poland was presented and met with approval.
Construction of extermination camps had already begun the previous fall, and mass extermination, mostly as part of 'Operation Reinhard', had began operation by spring of 1942. Roughly 2 million persons, nearly all Jewish men, women, and children, were immediately gassed upon arrival at Bełżec, Sobibór, and Treblinka over the next two years, when these "Reinhard" camps were closed and razed. More victims would meet their fate in additional extermination camps such as Chełmno, but most infamously at Auschwitz-Birkenau, where slightly over 1 million persons, mostly Jews, died. Under the plan set forth at Wannsee, exterminations were hardly limited to the Jews of Poland, but rather Jews from all over Europe were rounded up and sent east by rail like cattle to the slaughter. Although the victims of the Reinhard Camps were originally buried, they would later be exhumed and cremated, and cremation of the victims was normal procedure at later camps such as Auschwitz.
##The Camps
There were two main types of camps run by Nazi Germany, which is sometimes a source of confusion. Concentration Camps were well known means of extrajudicial control implemented by the Nazis shortly after taking power, beginning with the construction of Dachau in 1933. Political opponents of all type, not just Jews, could find themselves imprisoned in these camps during the pre-war years, and while conditions were often brutal and squalid, and numerous deaths did occur from mistreatment, they were not usually a death sentence and the population fluctuated greatly. Although Concentration Camps were later made part of the 'Final Solution', their purpose was not as immediate extermination centers. Some were 'way stations', and others were work camps, where Germany intended to eke out every last bit of productivity from them through what was known as "extermination through labor". Jews and other undesirable elements, if deemed healthy enough to work, could find themselves spared for a time and "allowed" to toil away like slaves until their usefulness was at an end.
Although some Concentration Camps, such as Mauthausen, did include small gas chambers, mass gassing was not the primary purpose of the camp. Many camps, becoming extremely overcrowded, nevertheless resulted in the deaths of tens of thousands of inhabitants due to the outbreak of diseases such as typhus, or starvation, all of which the camp administrations did little to prevent. Bergen-Belsen, which was not a work camp but rather served as something of a way station for prisoners of the camp systems being moved about, is perhaps one of the most infamous of camps on this count, saw some 50,000 deaths caused by the conditions. Often located in the Reich, camps liberated by the Western forces were exclusively Concentration Camps, and many survivor testimonies come from these camps.
The Concentration Camps are contrasted with the Extermination Camps, which were purpose built for mass killing, with large gas chambers and later on, crematoria, but little or no facilities for inmates. Often they were disguised with false facades to lull the new arrivals into a false sense of security, even though rumors were of course rife for the fate that awaited the deportees. Almost all arrivals were killed upon arrival at these camps, and in many cases the number of survivors numbered in the single digits, such as at Bełżec, where only seven Jews, forced to assist in operation of the camp, were alive after the war.
Several camps, however, were 'Hybrids' of both types, the most famous being Auschwitz, which was a vast complex of subcamps. The infamous 'selection' of prisoners, conducted by SS doctors upon arrival, meant life or death, with those deemed unsuited for labor immediately gassed and the more healthy and robust given at least temporary reprieve. The death count at Auschwitz numbered around 1 million, but it is also the source of many survivor testimonies.
##How Do We Know?
Running through the evidence piece by piece would take more space than we have here, but suffice to say, there is a lot of evidence, and not just the (mountains of) survivor testimony. We have testimonies and writings from many who participated, as well German documentation of the programs. This site catalogs some of the evidence we have for mass extermination as it relates to Auschwitz. I'll end this with a short list of excellent works that should help to introduce you to various aspects of Holocaust study.
##Further Reading
Check out the subreddit's Holocaust Denial FAQ, which has many fine entries, especially what /u/Elm11 wrote in the ELI5 about Holocaust Denial and the work of /u/commiespaceinvader in this 'Monday Methods' post.