Assuming Switch shipments of ~10 million, I'd predict:
Mario Kart 8 Deluxe - 15.2 million
Super Mario Odyssey - 14.8 million
Breath of the Wild - 13 million
Super Smash Bros Ultimate - 11 million
Pokemon Let's Go - 9.5 million
Splatoon 2 - 9 million
Super Mario Party - 4.5 million
1-2 Switch - 3 million
Mario Tennis Aces - 2.5 million
Kirby Star Allies - 2.3 million