I'm not really stepping into this much but all I can say is that the figures make sense to an extent. 3 million polish jews disappeared in the span of 6 years, about 2 million disappeared from the record in the USSR, hundreds of thousands of Romanian Jews, Hungarian Jews, German Jews, etc. There were camps, that is established. there are eyewitness accounts of conditions from these camps and from people who've seen the end result. there are also accounts of Einsatzgruppen, and I have personal accounts from people I've met who've described how their villages were wiped out by the Germans. Finally, you have a stated motive by a group of people who wanted to exterminate the Jews from the face of the earth.
Now, is the 6 million dead figure of just dead jews proven beyond a reason of a doubt? probably not, but there is a significant amount of evidence of Nazi atrocities perpetrated against Jews, you have a loss of six million lives, and you have a motive. By that alone, the actual number is only relevant because the autists on both sides hang onto that like it's the Ark of the Covenant.