It's an estimated figure but probably accurate. We're also looking at another 5 million or so people who were socialists, Russians and other communist nationals, intellectuals, handicapped, gay and lesbian, political dissidents and even Turks who looked "Jewish". Nearly every gypsy in Europe as well. In total the estimate is 11 million.
Concentration camp records were kept well and so as far as those who were in those and not dead of disease, beatings or being shot and killed in the ghettos, mass shootings, death marches, etc, there are some very clearly accurate estimates of how many entered and died in those. Those also contained the other minorities listed above, though usually they were kept separate or in different locations. Socialists, communists, Russians and Poles were considered sub-human.
That's the nightmare it was.
Add to that all of the deaths of military and civilians across the globe and it's horrifying. There were also biological warfare experiments and death marches, etc that Japan carried on against the Chinese. The aggressors who undertook to conquer the world can complain about many things but their plans to undertake this kind of destruction on people who weren't even their enemies, living in non-aggression with their neighbors prior to the war, is symbolized by that highest symbol of what they engaged in: The Holocaust. Japan also conducted biological warfare and death marches with the Chinese.
Let's hope people can avoid any additional world wars because nobody had nukes until the very last of WWII. It's too easy now. There won't be time to for the right, or try to correct the situation if the wrong hands are on the buttons (despite the SIZE of the button).