I would say around 6 millions, which would still be a lot. Not everything is going to turn into 10 million + seller.

I also think that Mario Maker will suffer from being a level builder game and not a "New Super Mario Bros". I don't think that making levels has a broad appeal. It will also release in June and will be forgotten by the holiday with Pokemon, Animal Crossing , Link'sAwakening, taking all the spotlight.

Ok. Do you also think Link’s Awakening will sell more than MM2? I think MM2 will reach the 10 million before the end of the fiscal year. Also, a 2 million increase from the original is really small 

I think LA will sell around 4-5 millions. MM on Wii U was the only major release at that period, I think it won't have the same hype on Switch.

Updated with latest figures

AC - N/A

Splatoon 2 - 190k left

MM2 - 4.96M left

Gen8 - completed

Gen8/Remake - N/A

Gen9 - N/A

SMP - 880k left

BOTW2 - N/AS

NSMBUD - 4.15M left

SMO - N/A 

New candidate is LM3.

First post should be updated with a summary and a beautiful list with colors (different colors for sup-20M and sup-10M)

So, it looks like we have already have 6 games that have crossed the mark.  Two games that are very close.  Three more games that are 5M+, and Animal Crossing just a couple of months away.  Considering we haven't even had a Direct yet from Nintendo yet in 2020, they might just get those 11 games at 10M+ by the end of this year.

10M+
Mario Kart 8 D  (22.96M)
Super Smash Bros Ultimate (17.68M)
Super Mario Odyssey (16.59M)
Zelda: BotW (16.34M)
Pokemon Sword/Shield (16.06M)
Pokemon Let's Go (11.76M)

Very Close to 10M+
Splatoon 2 (9.81M)
Super Mario Party (9.12M)

Over Half Way
NSMBU Deluxe (5.85M)
Luigi's Mansion 3 (5.37M)
Mario Maker 2 (5.04M)

Stong Unreleased Contender
Animal Crossing

Nintendo should at least have 1 unannounced AAA title during the holiday period. 

Either BOTW2 or the next 3D mario or the new 2D mario. 

Got this spot on, Splatoon 2 should be 10m with in the coming weeks Luigi's Mansion 3 I always fancied to break 10m while SMP will hit 10m possibly by March, Tomodachi was a possible future announcement I can see hitting 10m as well now that AC is arriving to wake that crowd up. BOTW's sequel is very likely to join this club especially if pushed while NSMB is looking to do the calm walk to 10m or more LT.

Switch should end up with at least fifteen 10 million sellers.
Done:
1. MK8D
2. SSBU
3. SMO
4. Z:BotW
5. Pokemon S/S
6. Pokemon Let's Go

Almost there (easily reaching this year):
7. Splatoon 2
8. Super Mario Party

Guaratneed from a known future game:
9. Animal Crossing

Possible already released games:
10. Luigi's Mansion 3 (likely)
11. NSMBU:D (maybe)
12. Mario Maker 2 (maybe)

Likely future announced games:
13. BotW2

Likely possible future games:
14. Another Pokemon
15. At least one more Mario 2D or 3D

And of course it's possible that there will be a 2D and 3D future Mario games released that both hit 10 million, adding one more to the above list. Possibly a fourth Switch Pokemon as well (though I'm really not sure about that as next Pokemon should be in a couple years and a couple years after that Switch 2 will likely be out). Maybe there will be another mega 10mil seller from a Nintendo franchise that hasn't come out on Switch yet. And maybe some game we don't even expect or god forbid a third party brings a big new super popular AAA game or even say like Ring Fit continues to have legs for years and creeps past 10 million in a few years.

So I'd say minimum is 15 with the max 18-20.

For some reason, I have strong doubt that NSMB, SMM2, and LM3 make it to 10 million.