Mario Kart has 30 million on lock.
Smash will continue selling until the Switch is dead so its pretty much guaranteed to hit it as well.
Zelda BotW is still THE killer "app" for the Switch and also will continue to put up good numbers until the Switch is out of stores, so I think it'll just barely creep over 30 million.
AC is a big question as we need to see how its legs are a year from now, two years from now, but I think it has a decent chance as gamers and casuals seem to buy it and love it in droves.
Mario Odyssey is dead even with Zelda now but has a bit smaller legs so I think it'll probably fall 2 or 3 million short. Pokemon had an insane holiday launch but fell off badly in its second quarter, I doubt it's legs will carry it another 13 million and assuming more critically acclaimed Pokemon comes out in a couple years I think that'll cut off any chance at 30 million and it'll fall several million short. The only possible future games I could see maybe hitting it are a brand new critically acclaimed 2D Mario game, a Wii Sports remake upgraded to perfection and sold at a budget price (probably won't happen tho), or if Game Freak blows the next Pokemon out of the water and totally revolutionizes the series so that it does even better than Sw/Sh (this is extremely unlikely tho).
So looks like I'm gonna say Switch will have four 30 million sellers when it's all said and done: MK8D, Smash, BotW, and AC.