You might be interested in the historic debates around the Holodmor that I describe here.

As for the "100 million deaths caused by communism" - as far as I am aware, this claim specifically comes from the Black Book of Communism, published in 1997 by Stéphane Courtois et al. While Courtois and the contributors are academics, I have to say that at least in English-language academic writings (the book was originally published in French) on the Soviet Union, this isn't a work that is cited much, if at all, and definitely not authoritatively. Courtois draws a lot on the totalitarian school, which supposes that fascist and communist regimes were effectively similar, "totalitarian" regimes, and this doesn't really have much currency in academia any more, at least in Soviet studies.

The book, from what I know, also suffers from a methodological flaw in that Courtois pretty much tried to work backwards to justify his 100 million dead statistic. It's also worth pointing out that the vast majority of the deaths that are included in those figures come from famines, specifically the Chinese famine during the Great Leap Forward (1958-1961) and during 1931-1933 in the USSR. In the case of the latter, as I note in the link answer, while most historians agree on the ultimate responsibility of the Soviet government, there are still debates around the intentionality. There's also a larger question to be asked around if the responsibility for such famines is different from, say, that of the British government in the Bengal Famine of 1943, but I am not an expert on that particular event.

In the broader sense, I would say that there is in particular a debate around the numbers of deaths caused by these famines, and that's not really something that can ever be completely settled. Malnutrition and starvation can obviously be fatal, but often is not a direct cause of death - these states weaken the immune system and allow victims to succumb more easily to epidemics, or can contribute to severe health problems later in life. Any estimates as to death totals will be just that - estimates, no matter how reasonable or plausible. As Ian Johnson notes regarding deaths in the Great Leap Forward, there is a mid-range estimate of 30 million dead, but other scholarly estimates range between 25 and 45 million. Johnson notes that both of those high and low end estimates are "credible" and "plausible", and perhaps they very well are. But they are still estimates and ones that differ by 20 million at that.

Even in terms of Soviet deaths, 20 million total dead gets touted, and is the figure used in Black Book. It's something of a "safe" high number that still gets used, and probably owes most of its fame to use by Robert Conquest, but isn't really supported by the academic literature that has been produced since 1990. Stephen Wheatcroft and Michael Ellman, who (as noted in the linked answer) argue quite a bit over the intentionality of Soviet famine deaths, both largely agree on the number of deaths from Soviet "repression" and famine to be substantially lower, and more in the 9 or 10 million range (as noted by Timothy Snyder here).

I think it's worth mentioning overall that these victims really are better thought of as victims of particular regimes, rather than of "communism". In the case of Soviet deaths, attributing the victims' deaths to "communism" is perhaps accurate in a 30,000 foot view sense, but does little to explain why almost all of these deaths occurred between 1928 and 1953, and almost none occurred in the latter half of the Soviet Union's existence.

In general, I dislike the focus on tallying these sorts of numbers. As can be seen, they tend to be used to create a balance sheet for political and ideological purposes. Whatever the estimates tend to produce, in most of these cases the answer will be 'millions", and we need to remember that these are individual lives we're talking about, and any amount of wrongful death is tragic and wrong.