For example, if somebody says "Holocaust victims numbered in the thousands, not millions," what would I say or show them?
Why are you wasting your time arguing with holocaust deniers? Do you also take part in group discussions about whether or not the sky is blue?
Keep your sanity, and avoid the topic all together.
I'm afraid I don't have any sources to contribute, mostly because attempting to "convince" Holocaust deniers is largely a fruitless labor, though arguably a necessary one. I've studied both Holocaust histories and conspiracy theories fairly extensively, and the problem lies in irreconcilable differences in how conspiracy theorists and Holocaust deniers define a "reputable source." If you show them extensive scholarship published in peer reviewed journals in multiple languages, hundreds of primary sources, and even contemporary news sources, which to anyone on this subreddit would check out as legitimate and convincing, to conspiracy theorists reputable sources are PART OF THE CONSPIRACY. They've been forged, corroborated by the government or other interested factions, circulated by the mainstream media, perpetuated by elite groups of scholars who are all conspiring to "hide the truth." Anything that you show a conspiracy theorist or Holocaust denier will become part of the conspiracy, and they will use it to aid their own argument instead of yours.
It's a completely different paradigm about what is reputable and legitimate. To someone like that, the most underground, unconfirmed, and shoddily written blog posts on a conspiracy theorist website would be more legitimate than the most well-researched scholarly monograph with hundreds of closely analyzed primary sources. Their views can only be confirmed by people like them, with the sources THEY choose, and any sources you present to contradict them will only be turned against you.
When it comes down to it, they don't want to be convinced. I've unfortunately had quite a lot of experience with Holocaust deniers, but I wish you the best of luck.
fyi, you may find something useful in the FAQ (link on sidebar)
From my comparitively brief time arguing with Holocaust deniers - there isn't going to be any one piece of evidence or source that will defeat Holocaust deniers. In practical terms this isn't because there isn't one good piece to use (there is stacks of excellent scholarship) - its because the denier will shift the argument to something else if you 'catch' them on any one point. Tell them their science is junk and they'll want to dispute the meaning of terms used by officials. Get them on the word game and they'll want to talk about discrepancies and hearsay in survivour accounts. It can go around and around forever.
Deniers will often try to 'refute' several facts relating to the history of the Holocaust (or at least what they claim that history is... strawmen are easier targets and require research) in a sitting, rather than stick to one key point. So straight off you'll have many things to respond to, not one simple weak point. The denier hopes to shore up their arguments through volume and the assumption that something this 'detailed' must be well thought out. Similarly denying several 'points' at once makes arguing with a denier more time consuming for anyone that might want to engage, which serves as a deterent to debate.
If I had to point to one common flaw in denialist arguments, it is that generally all they do is dispute. Most of the denial literature is about specific topics (a particular aspect of the Holocaust, a specific bit of evidence etc.) - a 'full' counter narrative is very rare. David Irving's "Hitler's War" is probably an example of trying to build a counter narrative I guess? What I'm trying to get at is the deniers don't really prove their arguments, only attempt to disprove other people's work. If you could somehow force deniers to actually build a narrative (speaking hypothetically here) it would be simple to point out the lack of actual support for their counter-narrative, logical flaws etc. Going back to Irving again, his work was savaged subsequently for misleading use of sources, presumably because Irving couldn't actually find enough evidence to satisfy his own claims. It would also preclude the tactic of changing the argument - basically switching the roles normally played in these arguments.
It's been said that if we take the thought experiment that the holocaust was a fiction, some of the many men executed for it would have denied it in their trials. Instead, German officers largely quibbled about exact numbers, responsibilities, and following orders - but never that it occurred.