I'm not too sure either, hence why I labeled it as 50/50. Luigi's Mansion 2 was at 5.75 million be the end of December 2017. Furthermore, Luigi's Mansion 1 was at 3.33 million be the end of December 2014:
https://www.resetera.com/threads/nintendo-software-and-hardware-sales-data-from-1983-to-present.2725/ So, I'm guessing that it's getting close to 6 million at this point. 
So, that's a jump of around 2.5 million from the 1st to the 2nd game. So another leap like that for the 3rd game would bring it to around 8 million. I think it has a reasonable chance for two reasons. 1) The trend with other Nintendo franchises getting quite the jump in sales from their previous system to the Switch. 2) It's a Mario-related game, figuratively and literally because it's Mario's (far more interesting) brother, and just about anything that has Mario's name or likeness attached to it on a successful system like the Switch is guaranteed to sell huge numbers. Time will tell though. It would certainly help a lot if the game is released and given an overwhelming positive reception and critical acclaim to help boost its legs after launch.

I'd have to say yes, at this point. Given how strongly key Nintendo franchises are performing, it's a good bet.

Already over 10m:
Mario Kart 8 DX, Super Mario Odyssey, Smash Ultimate, Zelda, Pokemon Let's Go

Should hit 10m, probably in 2019:
Splatoon 2, Animal Crossing, Pokemon Generation 8

Could hit 10m:
Super Mario Party

We're up to 8 practically guaranteed 10 million sellers and a possible ninth in the shape of Super Mario Party just by counting games already on the market or already confirmed. I'd expect Super Mario Maker for Switch to have a good shot at 10 million too, plus a Let's Go Johto version in 2021 to take it to 11. It's difficult to know beyond that - but sequels to things like Odyssey (or simply a new 3D Mario) and another version of Mario Kart might do the trick, depending on how long Nintendo keep Switch going.

Before coming here everydays, I had no idea about the importance of Animal Crossing. Can this game make 10+ easily ?
Otherwise I believe that we will have this at least :

-MK8D
-SSBU
-SMO
-Zelda BotW
-Mario Party
-Pokemon Pik
-Pokemon Eev
-Splatoon 2
-Pokemon core
-Animal Crossing (?)

What else could be a 10M+ ?

Splatoon 2 for sure, Pokemon 8 too, maybe Super Mario Party.

Metroid Prime is pretty niche but can make surprise numbers so why not

AC mainline games sell 10m, the last 2 mainline games sold 12m.

I was not imagining that it was so huge..I was lazy to check the charts for previous consoles. So if they can plan their Q3 with AC in October, Pokemon in November, + a 3rd strong one, they should surpass 11M of switch sold for Q3.

Of course it will. There's probably two games already out that will eventually hit it (splatoon and mario party). We know of two games this year that will be 10+ million sellers (pokemon, animal crossing). That's 9 right there. Throw in at least one more pokemon game, an actual new 2D mario game at some point, another 3D mario, and that's 12. And we will probably see a couple other games beat it as well. I think 15 would be a great goal to hit.

I'd say it will be more difficult at this point not to reach or pass those 10 10M sellers

2 are already well on their way (SMP and Splatoon 2), with NSMBUD potentially also breaking that number if it sells steadily over time.

2 Games coming out this year which should do it without much fuss (Pokemon and Animal Crossing), with Fire Emblem possibly also passing that number over time if it's very well recieved.

Next, among third party games, Minecraft should be able to reach that if it continues selling steadily, same could potentially go for Stardew Valley, Rocket League and some other Indies which always seem to be on top of the monthly download lists.

Finally, there's the question if free-to-start games like Fortnite count. If they do, then Fortnite and possibly Paladins could do it, too

Yes it will.

Confirmed:
- Mario Kart 8 Deluxe
- Super Mario Odyssey
- Super Smash Bros. Ultimate
- The Legend of Zelda: Breath of the Wild
- Pokemon Let's Go Pikachu/Eevee!

Guaranteed:
- Splatoon 2
- Animal Crossing
- Pokemon Gen 8

Most Likely:
- new 3D Mario
- new 3D Zelda
- Mario Maker 2

Somehow possible:
- New Super Mario Bros. U Deluxe
- Super Mario Party
- Minecraft
- GTA V (if comes to Switch)