Only games I can see doing it at this point are the Diamond/Pearl remakes, Odyssey 2, OR BotW 2.

The next mainline Pokémon is the only surety as I don't think MK9 will happen this gen BotW 2 and Odyssey 2 could possibly reach 20m but I'm expecting them to both be down on the first games but still hitting 15m which is a great feat on it's own. There are plenty of potential games left that will hit the 10-15m mark but I don't think there is much more that will push on past 20m.

A Switch-exclusive Mario Kart could do the trick. However, I don't see any reason why Nintendo would actually make one considering how well MK8D is selling despite technically being a port.
Other than that, another mainline Pokemon title could do it later down the road (2021-2022).
Also, a new classical 2D Mario title should be able to do so. And unlike with MK8D, I don't think Nintendo is satisfied with NSMBUD and/or SMM2 sales to let that opportunity pass.
Now this may be more out of left field, but how about a successor to Tomodachi Life, maybe?
Finally, maybe Ring Fit Adventure could reach it in the end. It doesn't look like it's sales were to slow down anytime soon, and shipments still sell out more or less as fast as they come in.

At this point anything could be a potential 20m seller. Remember that before the Switch, Zelda, Smash, and Animal Crossing wouldn't have been considered potential 20m sellers.

LM3 should reach 10mil.

Pikmin4 would be huge if it did 3mil as original did less than2mil

Style Savvy

Lock:

MK8D
Smash
Odyssey
Breath of The Wild
Sword & Shield
New Horizons
Pokemon Gen 9 (if on Switch)

Very Good Chance:

BotW2: I'm assuming every sequel sells less than its predecessor. If BotW2 is as good as the first, I think it should do it. If it manages to be even slightly better, it will definitely do it. Don't forget about the 1.5-2M people who bought it on Wii U and will likely get the sequel.

Less Likely, But Possible:

Diamond and Pearl Remakes: Let's look back at the DS and 3DS. On the DS, Diamond and Pearl came out first and sold the most. Black and White sold less, and Heart Gold & Soul Silver less than that. Same trend on 3DS. X&Y>Sun & Moon>Omega Ruby & Alpha Sapphire. So from that I expect Gen 9 to sell less than Sword & Shield, and D&P remakes less than that. Therefore I think it's unlikely that they will hit 20M, but I do consider it a possibility given how much Sword & Shield surprised me.
Super Mario Party: I wouldn't have guessed, but with it being at 9.12M already, it could. Seems like the kind of game to have decent legs.
Splatoon 2: I actually would have considered this a greater possibility not long ago, but it seems like sales have slowed down. But with it being half way there, I certainly wouldn't rule it out.
Odyssey 2: I originally had this in the Very Good Chance Category, until I looked up the sales of Galaxy 2; about half the original despite its critical acclaim. So never mind that.

Also, I don't understand why so many people seem to think MK9 will come to the Switch. There's really no reason to think so.

It's also worth noting that the Wii U version sold around 2m, so it's already very close and has possibly already hit 20m.

How many 10mil sellers will be on the NSW?

More than on any other Nintendo console by far.