The question now isn't "how many" but "3rd party or not". They're having a dream-like 1st year in 1st party titles and that has been quite effective. But even if they keep releasing stellar 1st party titles in 2018 and beyond, that won't be enough to keep hardware sales high as all the people potentially interested in Nintendo-like experiences will have the system already. The only thing that can ensure strong momentum is major 3rd party exclusives but they aren't going to have many.

Btw, it's worth noticing how weak the Switch is in Q1 2018 for both exclusives and non-exclusives : they are like 30 PS4/PSV games that have a date/window up to Spring 2018, but Switch has like half a dozen in the same timeframe. I don't see how sales can keep up with PS4 in this situation.

@OP, do you agree with this? If you say 'major title', are you talking about games like Fire Emblem or games like Yoshi and Kirby?

There's a difference between games that Nintendo fans are exited for, and games that sell well. For example, Xenoblade 2 won't outsell 1-2-Switch, despite  being more hyped.

Confirmed for 2018:

- Kirby
- Yoshi
- Fire Emblem

The only thing I'm hyped for on that list is Fire Emblem. So hopefully we get to see Retro's new project (+ launch in the same year).

There's also some good third party exclusives that will normally come out in 2018:

- No More Heroes: Travis Strikes Back
- Project Octopath Traveller
- Shin Megami Tensei V

It will sell better than it. Switch games are announced left and right and release pretty quickly. Do you honestly believe those 30 PS Vita games are going to interest anyone outside of an incredibly niche userbase?

Like this year, 4 seems like a good number. Replace BoTW, Mario Kart, Splatoon and Mario Odyssey with Animal Crossing, Smash, Mario Party (hopefully back to Wii/DS levels of sales) and a 2D Mario and you're set for 2018.

I fear that Nintendo may have blown their wad too early.

We won't be seeing another Zelda or Mario for at least another two or three years.

As many as they can get, really.
1st Quarter: BOTW, MK8D
2nd Quarter: Splatoon 2, Mario and Rabbids, ARMS
3rd Quarter: Mario Odyssey, Xenoblade 2, Fire Emblem Warriors (and a bunch of third party ports ala Rocket League, Skyrim, DOOM...)

4th Quarter: ????

(Mario Odyssey and BOTW heavily carrying it, here).

So, it's pretty much as many as they can get. If we skip ahead to 2018, to maintain popularity they need to announce a new game for a 2018 release date that is on the verge of BOTW/Odyssey popularity, if not miscellaneously Kirby, Yoshi, Fire Emblem, No More Heroes, etc. A good E3 could definitely do justice, but I;m convinced that this year will be better for first-party Switch games than the next. Next year, hopefully, it will be carried by third-party developers. The next "big" first party title in terms of maintaining popularity is definitely Pokemon. It's probably 2019 though.

when you say "major exclusives" i think in Mario 3D-2D/The legend of Zelda/Metroid Prime/Pokemon/Animal Crossing/Splatoon/Super Smash Bros/ Mario Kart (and third party: Yokai Watch and Monster Hunter). Other first party games sell well but are not "Major exclusives".
Nintendo needs to 3 games exclusives each year with good score (+91) in Metacritic/ the Gamers

Disagree, we can see top down Zelda in 2 years even 3D Zelda in 3 years, we could easily see 2D Mario next year or new 3D Mario in 3 years. You need to consider that this time with Switch resources for games will not be separate on handheld and home console, they will suporting just one platform, Switch.

I would actually rather ask how many 3rd party exclusives they need. Nintendo has never had any issues with any of their consoles when it comes to first party lineup, but as good as Nintendo is at making games they can't fill a console's library by themselves.

Nintendo can generally be trusted to bring out at least one or two big titles each year, most of which will likely be very good, but very often the problem has become filling the gaps between Nintendo's own big titles. It's not an issue yet, especially with how great the games they've made so far are, but in the long run the Switch will need to have big exclusives that aren't made just by Nintendo.