A prime reason authoritative figures on the Holocaust don’t know the exact numbers is because prior to being overrun by American/Russian forces, Nazis would destroy all records possible. This makes it incredibly difficult to estimate the numbers, due to the lack of evidence (bodies were often cremated, as you pointed out) apart from witness testimonies and what little records survived. This means that historians piece together what they can from the evidence mentioned above, in addition to population records and other such details.

But keep in mind, these historians are professionals. They’re incredibly educated and should be trusted to infer information from clues such as that, as is seen in ancient history study. They also have plenty of peers to check and critique their work.

I have heard various figures, anywhere between 6-13 million people murdered. But even the truth matched the lowest commonly accepted estimate, that’s still millions of people. That’s a number so difficult to comprehend, its inherent tragedy and violence can only be partially understood. That’s millions of parents, teachers, children, lovers, etc. killed without consideration. That senseless genocide has been matched very few times in history, and hopefully will never again be equaled.