In Babi Yar, over two days, 33,771 Jews were killed, and this was prior to the 'peak' in Operation Reinhard:

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

The Nazis were still killing people in other places at the same time, so the deadliest day is probably much much higher.

The scale of the Holocaust is hard to imagine. Even just looking at very specific suranmes, there are 23,000 killed with the surname Rosenberg, 12,000 with the surname Adler...

https://collections.yadvashem.org/en/names/search-results-na...