That’s an extremely hefty prediction. It would have to outsell every other 3D Mario by at least 7 million units. Galaxy 2 was one of the most critically acclaimed 7th gen games, and it didn’t even come close to that. Time will tell.
On a side note, I distill dont see many Switch games having a high digital attach ratio. I think for this game, it’ll be 10% at best, so you’re talking at least 18 million shipped/ sold at retail, if I’m not wrong.