My recollection is that number came from Eichmann at his trial. The actual number of people killed is probably around 11 million, with about half that number (around 5.5 million, give or take a few hundred thousand) being Jews. So '6 million' is a rounding up, but a reasonably accurate number for general discussion of the subject of the extermination of Jews.
Okay, but do you have a source? And why do the other 6 million people not seem to count?