While I saw a lot of answers for the original topic and many of those games I think would pass ten but not twenty million, I will say a game I believe has no chance of hitting twenty million but could go past ten million on Switch (if released exclusively):
True. If they actually bothered to release a legit brand new Monster Hunter on Switch I think it absolutely would hit 10 million. I just doubt they will do that as the go to strategy for all third parties seems to be put old stuff or side projects on Switch. Which is bizarre considering its popularity but whatever. Would love to see a new Monster Hunter made from the ground up with their A team for the Switch.