The numbers are pretty accurate, the Jewish population of Europe pre-war was a known quantity, after the camps were liberated the surviving Jews went to DP camps, those people were counted. Pretty simple math - (European Jews Before the War) - (????????) = Jewish Survivors Counted After War
Nobody knew about the death camps, nobody could imagine such a horror, until very late in the war.
I read an incredible story about 2 Czech teenagers, who escaped one of the death camps against all odds, and their race to tell the world about the horror.
edit - Slovak not Czech
http://en.wikipedia.org/wiki/Rudolf_Vrba
Rudolf "Rudi" Vrba (11 September 1924 – 27 March 2006) was a professor of pharmacology at the University of British Columbia. Originally from Slovakia, he is known for his escape, at the age of 19, from the Auschwitz concentration camp in German-occupied Poland during the Second World War, and for having provided some of the earliest and most detailed information about the mass murder that was taking place there.[1]