I was definitely wrong about Super Mario Party not quite making it to 20m this year but I still think that TOTK may get to that number before the SMP joycon bundle arrives:
https://gamrconnect.vgchartz.com/thread/247334/will-zelda-totk-make-history-for-the-switch-this-year/1/
Either way, both will be over 20m by the end of the year for sure and with Super Mario Bros Wonder, the Switch will have ten 20m sellers.