Hmmm. Zoat is probably right. Even accounting for their natalist policies, that first estimate is probably a little low. I still think around 3 Billion is probably accurate for the scenario of them making the entire planet as white as possible.
But since they still have allies among certain groups/nations in at least this fic's version of Earth 10 . . .
The biggest dip in population would probably be them purging black people from Africa. Because I don't think they'd they'd merely deny aid to African peoples. When these Nazis are conquering Britain and launching Trans-Atlantic invasions that end with the successful conquest of the United States?
I definitely think they'd basically take Sub-Saharan Africa away from the defeated French and British. Get to work on their idea of a Mittelafrika.
Just assume they also grab Liberia and the rest of South Africa. Especially South Africa. That has good Germanic stock in the form of the Boers that they can work on integrating. (They planned to buy Mozambique and Angola from Portugal, just FYI.)
Oh, and more than likely let Italy have Djibouti so it can be added to Italian East Africa.
The Nazis considered black people to be on the same level as Jews, Slavs, and Gypsies. So you can imagine just how badly Africa would be depopulated.
Interestingly enough, despite this, they weren't really sent to the camps because there were so few blacks in Germany that they didn't really consider them enough of a threat to do much more than sterilize some of them. Though they did still face quite a bit of racial discrimination.
I'm sure this would change after a total victory, and that they would have ended up in the camps as well in due time.
ANYWAYS . . . While that would ordinarily result in a smaller population for Earth 10, the Nazis really, really wanted German women to pop out a lot of babies. To the extent that the created a special medal to honor German women with lots of babies. A medal that came with a boatload of special privileges.
https://en.wikipedia.org/wiki/Cross_of_Honour_of_the_German_Mother
If you don't feel like reading . . . They were pushing for every German family to have 4 to 8 kids.
So yeah. Earth 10 is probably above 8 Billion now that I've thought about it more. With between 2 and 3 Billion of that being white, and with black people being considered endangered.