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.

Leaning more towards the 25-30 million range but wouldn't be surprised if it crosses 30 million.

I think it'll sell over 25mil even if SMO2 releases on the Switch. 26-28mil without Nintendo Selects or a big permanent price cut.
30mil+ is possible with that.

Under 30 million, probably right around what Sw/Sh will do. If Nintendo did the Select's discounts that would help, but they are probably gonna wait to do that until like right before the Switch is replaced so it won't make much of a difference. I could see it ending up over 28 million.

It's likely to be over 30mil for the reason that switch is selling well.

Similar to the topic in the sales thread about sales of the console itself, I think it's hard to make calls on this without knowing how long Nintendo plans to support the Switch for. If they phase it out in the next couple of years then SMO will have a hard time cracking 30m, but if it keeps getting supported into 2023-24 and if Nintendo does a Selects line, then 30m seems very achievable.