John Nowak
Graduate Poster
If I recall correctly, depending on the historical scholarly research source looked at, the estimated number killed ranges from a low of 4.8 million to a high of 5.7 million.
IIRC, the earliest back-of-an-envelope lowball estimate came up with a figure of 5.5 million. Under those circumstances, rounding up to 6 is entirely legitimate. Later estimates would vary, naturally enough.
Thing is, I can't recall where this came up. John Toland's biography of Hitler?

but thank you!