I doubt if anyone has a definitive answer.
Millions of people died during and after the war, millions of people moved around from country to country to avoid the fighting.
The Germans themselves moved people from camp to camp and country to country so keeping track of anything was difficult.
Many people disappeared during the war and are probably buried in unmarked graves. Maybe they were killed by the Germans, maybe they died in an explosion. Who knows.
Although the Nazis killed Jews they also killed other people like Gypsies, Homosexuals, disabled people, mentally ill people and so on. Do you count them ?
Some Jews and others were taken to do forced work (on Jersey building concrete bunkers for example) and died while there.
Some people starved to death while in camps, others were deliberately kiiled.
Also some things went on behind the iron curtain that have never been revealed. Land occupied by the Germans was taken over by the Russians after the war so details of what went on there is difficult to find.
In fact there are stories of Russians killing people (like Poles) near the end of the war so how many people were killed by the Russians but the blame is put on the Germans.
So all in all, any figure is just an educated guess.
Best just to say something like "between x million and x million"