Five (in no particular order)

1. Breath of The Wild
2. 1-2 Switch
3. Splatoon 2
4. Mario Kart 8 Deluxe
5. Super Mario Odyssey

I'm not too sure about Skyrim crossing the million mark.

Sure, it's a very popular game, but it's also 6 years old, and already available and cheap on PS4/PS3/360/Xbone/PC.

It's something of a wild card the way I see it, could go either way.

Zelda is already a million seller, Mario Kart 8 DX is at ~740k based on one day of US and three days of Japanese sales, and by Nintendo's own statement, 1-2 Switch is almost there. I would say at this point we have three confirmed million sellers.

It's tough to decide how many more there'll be. Out of the games announced at this point, I would say Super Mario Odyssey and Splatoon 2 are certain to hit 1 million, and Arms and Fire Emblem Warriors both have a good chance. If Smash Bros and Pokemon Stars come out, then there's two more that are practically guaranteed million sellers. Depending on when they release, third party titles like Minecraft, Dragon Quest XI and Skyrim might have a shot too. If Capcom bring a Monster Hunter title out by the end of March 2018 on Switch (they have confirmed a major title will be launching by then, no format announced), then we would have another million seller in all likelihood.

So I'll say 10 plus, with 3 or 4 third party titles joining 9 million selling titles from Nintendo. Some of Nintendo's titles (Mario Odyssey, Zelda, Splatoon, Kart, Smash/Pokemon if real) could all be in the 3-5 million range if Switch does well enough, too. Hell, Zelda basically already is.

Pretty much these, ARMS could be a surprise hit and possibly 1-2 Switch (surpringsly enough) or bomberman R

https://mynintendonews.com/2017/05/04/mario-kart-8-deluxe-passes-one-million-sales-worldwide-in-its-first-weekend/

Mario Kart 8Deluxe sold a million already. Updating list.

I put 9-10.

Zelda
MK8D
1-2 Switch
Splatoon 2
Mario Odyssey
Bomerman R
Arms
Minecraft

That's 8 right there. What else is coming out? Xenoblade, Fire Emblem? Probably at least a couple we don't even know exist yet. My guess was probably low.

The rising tide lifts all boats. When the software sells like thing (due to hit titles), it makes people want to buy more games. People bought the system for Zelda but were also picking up Bomberman and 1-2-Switch with it as well. I'm sure this has also helped Puyo Puyo Tetris. The Games that will break 1 million:

Zelda
1-2-Switch (On track according to Nintendo)
ARMS - (thinking 2-3 million)
Splatoon 2
Mario Kart 8 Deluxe
Skyrim - (There is a constituency of open world games that will form on Switch. This will make games like Skyrim big success. Also portability)
Minecraft - (See above)
Sonic Mania - (If you look at Nintendo's Youtube, it is one of the most popular videos from the presentation trailers. Over 4 million views. Has more than the official Sonic channel. This will be a surprise hit).
Super Mario Odyssey - (Most 3D Mario games sell at least 10 million)
Puyo Puyo Tetris (Think this will sell due to word of mouth. The demo was a great idea)
Sonic Forces - (Most Sonic games sell well on Nintendo platforms. I don't think this will be a huge hit, but it could break a million)

The last two may be a bit out there. Also, there will be more games at E3 that Nintendo hasn't shown. 10 games is doable.

Not quite most; 3/6 to date.

That said, I agree that Odyssey will pass 10 million, which will make it "most" again.

Fire Emblem itself is next year, Fire Emblem Warriors is this year.

As for Xenoblade, it's possible it might make it, but neither of its predecessors did, so it's not a lock.

Zelda
1-2
Kart8
Minecraft
SSF2
Arms
Spaltoon 2
FireEWarriors
Skyrim
Mario Odyssey

If Bomberman is really at 500k already this one too.

I voted 5-6 but I should have counted earlier.

Eight.
MK8 Deluxe is confirmed million-seller, so is Mario Kart 8 Deluxe. 1-2 Switch is apparently well on its way there, but we'll see.

I see six more in the form of ARMS, Splatoon 2, and Super Mario Odyssey; supplemented with Super Bomberman R, the Skyrim remaster (easy sales in the form of people drawn by Zelda), and maybe 1-2 Switch.
I know I'm being very optimistic, so I can always say that I'm swapping out one or two of those with surprise announcements from Nintendo from E3, though I really want the new Bomberman game to hit 1 million sales.