The names and places of more than 3 million Jews killed are recorded. Many are unrecorded because whole towns and regions were made Judenrein. Those without surviving relatives and friends are more difficult to individually identify.
Jews were carefully segregated and counted, for tax purposes if nothing else. The numbers before and after are pretty solid.
What I find curious, and somewhat amusing - is the strange character of those who hate the Jewish people, like Hitler, and yet deny the Holocaust. What then do they admire Hitler FOR?
Same mindset as those who hate America, love Osama bin Lauden, but deny that Al Queda was responsible for 9-11. What then do they admire 0sama bin Lauden FOR?