Pretty self explanatory.
Mario's first 3D outing on Switch really crossed the 20m mark, becoming the first 3D Mario to do so. While it has slowed down compared to other evergreens (notably, Zelda is outselling Mario), SMO is obviously not done selling.
How high do you think it can go?
Somewhat optimistically, I feel like it can go over 30m, specifically over 31m (so it passes NSMB DS & NSMB Wii). I feel like this can be achieved if SMO is a Nintendo Selects title, if there is no SMO 2 and/or if SMO receives DLC (the latter is not likely imo). Ultimately, I think it can surpass SWSH in the end.