If you are looking for a 'record' of sorts, at Belzec 434,508 Jews and an unknown number of gypsies were slaughtered, and there were only two (!) known survivors at the end of the war - Rudolf Reger who emigrated to Canada and Chaim Hirszman, who was murdered by Polish nationalists in 1946.
There was also Maly Trostinents, near Minsk, Belarus. It is virtually unknown, except among specialists on the Holocaust. The main reason is said to be that there are no known survivors at all out of about 50,000 victims taken there.