I'm guessing it'll be higher than 20 million, but not much more than that. So I went with the 19-20.9 million option.

Bonus Question:
Not on the Switch we will. Nintendo seems very content with just letting the Wii U Mario ports constantly sell well and rake in the easy millions they bring in. That's why we're not going to see Mario Kart 9 until Switch 2. We might still get a new 3D Mario game. But with 3D World Deluxe right around the corner, along with the 3D All Stars collection, I've gone from "likely" to "doubtful." Just like I think they're content with Mario Kart 8 Deluxe and New Mario U constantly selling, I think they'll be just as content to let 3D World do the same thing and let that game keep selling along with 3D All Stars (until the end of March anyways) and Odyssey.

The Switch is still going to get a plethora of new games, big and small, most of which I think will be 3rd party. But as far as Mario is concerned, B-C tier spinoffs like another Mario Party, Mario Golf, Mario Sports Mix, or another Mario RPG (Whether it's a new Paper Mario, Mario & Luigi, something new, etc.) is all we're going to get. I think the big, blockbuster, A-S tier stuff like Mario Kart and the platformers are being saved for Switch 2. I fully expect Mario Kart 9 to be a launch title and the next 3D Mario game within the first year of its cycle. Once Switch 2 or whatever their next system is comes around, THEN I think we might see a brand new 2D Mario game... Assuming they haven't just replaced the 2D line with Mario Maker.