So Switch is at six 20 million sellers, with Mario Party and Ring Fit both likely to eventually hit it as well, along with BotW2. Presumably a brand new 2D Mario game will come out at some point and should certainly hit that mark as well. Add in the possibility one one of the upcoming Pokemon hitting 20 million, of course possibly both but let's just assume most likely only one of them might hit it. Switch could end up with eleven 20 million sellers!
10 million sellers is becoming the norm for almost every major game. Luigi's Mansion is about to hit 10 (or already has), 3D World will easily hit 10, the upcoming Mario Party will hit 10, Splatoon 3 will hit ten, obviously both upcoming Pokemon will easily hit 10, Minecraft is probably at 10 by now, Monster Hunter Rise will hit 10, hell maybe even Skyward Sword will.