I'm not as conversant with that part of history, but here's what the US Holocaust Memorial Museum has to say.
https://encyclopedia.ushmm.org/content/en/article/documenting-numbers-of-victims-of-the-holocaust-and-nazi-persecutionIt estimates 5.7 million non-Jewish Soviet civilians were killed in WWII. A horrifying number, to be sure, but a rather smaller percentage than the two-thirds of European Jews, or the 90% of Polish Jews, who were killed.
For the Soviet military, numbers seem uncertain -- at a glance it's around 10 million. Neither of these, of course, count the mass murder Stalin was himself committing, and there the numbers get really uncertain. Some say 1 million during WWII, some say more like 10 million (with many more before and after, of course). Based on reading Solzhenitsyn and others, I'm inclined to believe the high numbers and consider the low ones one step above Holocaust deniers. It's much harder to get definitive answers, though, when the nation committing these crimes has decades to get rid of the evidence.
Among Stalin's great crimes, none of this counts the Holodomor, the mass murder by starvation of Ukrainians, as it took place in 1932-33.
What, did you imagine I was the stereotype of a Jew you had built up in your head, to shrink like a vampire before the cross of "other victims of Hitler", to burn in the holy water of "other genocides"? Just the opposite. The horrific mass murders of all sorts of tyrannical governments have long been an interest of mine. I have no problem acknowledging that in their own mass murders, Stalin killed more people than Hitler and Mao killed more than him, or that those genocides have their own deniers.