I voted 18 million
I expect this year will probably be like 21 million. Next year if Nintendo is aggressive I think they can definitely do 18 million. If they are less aggressive though it might be 16 or 17 million.
By aggressive I mean:
Next year they've got BotW2, presumably Advance Wars will finally come out, and presumably most of the other games we want/expect on Switch: MP4, old MP remakes/remasters, DK, Mario 2D, WW/TP pack, and more, their N64 library will be filling out and hopefully they'll add GB/GBA to the expansion pack service, presumably a price cut on all models, drop a bunch of older first party games to $30 with the Nintendo Selects branding. If they do all this I could see 18 million, and then in 2024 they could coast on third party releases and the final few first party games for the system, probably one more Pokemon game, and set themselves up for a big Switch 2 launch holiday 2024.
I don't think we'll see any more hardware models, but if they do release a "pro" model next year then obviously sales could be higher.