Where did the number six million come from?
The number seems to have first been mentioned by Dr. Wilhelm Hoettl, an Austrian-born official in the Third Reich and a trained historian who served in a number of senior positions in the SS.
In November 1945, Hoettl testified for the prosecution in the Nuremberg trials of accused Nazi war criminals. Later, in the 1961 trial in Israel of Adolf Eichmann, he also submitted to a lengthy series of questions from the prosecution, speaking under oath from a courtroom in Austria.
On both occasions, he described a conversation he had had with Eichmann, the SS official who had principal responsibility for the logistics of the Jewish genocide, in Budapest in August 1944. In the 1961 testimony, Hoettl recalled how “Eichmann … told me that, according to his information, some 6,000,000 Jews had perished until then -- 4,000,000 in extermination camps and the remaining 2,000,000 through shooting by the Operations Units and other causes, such as disease, etc.”
https://www.haaretz.com/jewish/holo...-6-million-where-is-the-figure-from-1.5319546
They counted all the legs and divided by 2.