Looking at the full list of titles, I think we're guaranteed at least six (6) 20 million sellers. We've already got one of 'em, and we may get another in the next quarter update.
Mario Kart is a lock for 30 million, it may even do it by the end of December. Not only do I think it can pass Mario Kart Wii's totals, but even the original Super Mario Bros. on the NES (40.24), and become the greatest selling Mario game, period.
Hardware Comparison Threads:
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017