Well, Mario Kart 7 sold 14 million units on a system that had 75 million lifetime total. Mario Kart 8 sold 7.5 million on a system that had 14 million lifetime total. Then we have to factor in that the 3DS had a low attach rate, due to people upgrading their systems. Splatoon did about 4 million on a system that sold 14 million lifetime. So, even if the Switch does poorly in the next few years only selling 75 million lifetime, we should expect 10+ million sales of Splatoon 2.
IMO the 5+ million selling games on the Switch will be...
Animal Crossing Switch
Mario Kart 8 Deluxe
BotW 2 (They have to make this. They just do!)