I think 11.5-13m, with 12-12.5m being the range I believe in the most. Though, honestly, it seems like whenever I make a positive prediction about a game I specifically like a lot, it goes wrong .
I think the expansion pack will boost the original game and make people buy it, either as a bundle or as the base game with the dlc being purchased later on, even if maybe not to the extent of the prior games (where their sales were doubled with re-releases). So if it ends it's first year with about 9m-10m sales, then I think it will gain a few million throughout the rest of it's life thanks to the marketing brought to the expansion along with just normal legs.
Link to the VGChartz Discord server: https://discord.gg/2uMTaYtv
Someday you will die somehow
And something's gonna steal your carbon