Holocaust Deniers try to shed doubt on the reality of the Holocaust by questioning the accepted number of victims, 6 Million. But how do we know that 6 million Jews were killed in the Holocaust?

Sources

https://encyclopedia.ushmm.org/content/en/article/documenting-numbers-of-victims-of-the-holocaust-and-nazi-persecution

https://www.haaretz.com/jewish/holocaust-remembrance-day/holocaust-facts-6-million-where-is-the-figure-from-1.5319546

https://en.wikipedia.org/wiki/Evidence_and_documentation_for_the_Holocaust

https://www.un.org/en/holocaustremembrance/docs/FAQ%20Holocaust%20EN%20Yad%20Vashem.pdf

https://encyclopedia.ushmm.org/content/en/article/evidence-from-the-holocaus