No, no historian thinks he will find the definitive truth. My point is that there are valid historians (I only know about a few fields, but you can see which historians are valid by how much their peers respect their work) that at least try to get as close to the the probable number of, in this case, deaths. One example: taking a look at Rummel's data on the Soviet Union, he reports 60 million deaths 1917-1987, which is laughable (he even claimed that was a low estimate!). That's the number reported by Solzhenitsyn, which is not considered a reliable source in contemporary Sovietology (probably not a source at all). Here is a relevant thread on r/AskHistorians if you're interested: https://old.reddit.com/r/AskHistorians/comments/3v5u2t/are_r...
I'm really sorry an AskHistorians thread is not good enough for you. Let me paste Jonathan Smele's remark about Rummel's work (Smele is known for his compiled bibliography on the Russian Revolution):
>A poorly researched, obsessively anti-Soviet polemical general survey. [1]
Unsurprisingly, you would have read that exact quote in the reddit thread I linked. Guess that was too much to ask for.
[1] Jonathan Smele, Russian Revolution and Civil War Annotated Bibliography
The point is that in this case Rummel doesn't even attempt to be objective. You can be biased, everyone is. A real historian, someone who is respected by his peers, actively tries to remove his bias from his work. It isn't really profitable (you earn much more by writing extremely biased pop-history) but some individuals, such as me, value it.
It's pretty hard to get objective and accurate count here; unlike Nazis, Soviets (and I imagine Chinese et al) did not keep meticulous accounting even of people specifically executed, let alone of those keeling over in prison camps despite technically being only sentenced to imprisonment). Solzhenitsyn himself said, at least initially, that his numbers are extremely approximate, based on what personal research he could do and that he hopes future historians will do better.
Having said that, 60 million killed in USSR alone would seem highly suspicious. 60 million repressed sounds quite feasible. And as Stalin would acknowledge (death of one is a tragedy, death of millions is statistics), in a way it doesn't really matter -- communists killed as many, if not significantly more, people as Nazis, but because they usually did it to their own population, and weren't defeated by an invading army, they escaped anything resembling Nuremberg Tribunal, and to this day Nazi defenders keep quiet in decent society, but Soviet apologia is pretty rampant.