Will do better than the WiiU if they actually stick to their plan of unifying the handheld and console output and stick to it, but not by much. The mainstream is more entrenched in mobile games than ever, the Switch is completely overprized and facing extremely strong competition in the form of Sony (and to a much lesser extend Microsoft) and the PS4 is dominating the market. PS4 is selling for much, MUCH cheaper than the Switch and the Pro is hardely more expensive, particularly in Europe (Switch is €70 less than a Pro in Germany and France, just lol).
PS4 and the Xbone have the cheap home console market completely covered and sell for much less than the Switch, especially during sales. Meanwhile the Switch is overpriced compared to them and that in addition to that the high prices of the peripherals will scare mainstream customers and parents away ("hmm, it is kinda pricy... how much do I have to pay so my other son can play with his brother?" "Either $70 for a pro controller or "110 for a full, complete Joycon set, miss"). Also lack of 3rd party support (there is some right now, but we know how it goes) reduces mainstream appeal even further, particularly if one begins to take the additional costs with peripherals, much higher game prices compared to WiiU and 3Ds and the lackluster required for online-play subscription. I doubt even Pokemon will be able to do much, one of the reasons why Sun/Moon sold so well was that people coming from Pokemon Go could simply pick up a very cheap 2Ds for $70/80 and the game, that ain't gonna with the Switch (hey, you even need to pay if you want to trade Pokemon now!). The portability won't have much impact on the mainstream that is already happy playing mobile games for the most part on a device everyone owns and has to carry around anyway.
On the other hand the console is completely overprized and outmatched from a hardware perspective, so it won't woo the hardcore crowd outside of the Nintendo faithful (who are not numerous enough, see WiiU). And that the Pro (and later the Scorpio) is in a completely different galaxy power wise while not being much more expensive and offering features like 4k and VR is just the final nail in the coffin for that target group.
The console will go the way of the early days of the 3Ds and unlike back then there is far more, cheaper and deadly competition on the market (back then there was... a struggling Vita), so they would have to do massive price cuts to make the console more popular and even then it's not guaranteed it will take off. Sure, things might look rosy for a while because the Nintendo Faithful are buying the little (intentionally understocked) stock there is (shown by everything selling out in barely a few hours), similar to the WiiU, but reality will come crashing down soon enough.
And even then, the PS4 has been steamrolling since the beginning. On the other hand the Switch has massive price issues, a non-existent release line-up outside of Zelda (which you can also play on the WiiU) and it takes months for them to even get their online (subscription) services running, let alone release 1st party games. They might also end up getting quite a bloody nose this year, which will hopefully bash some sense into Nintendo's management.
So to sum up the wall of text, my expectation is that will sell more than the WiiU, simply because Pokemon and the portability will have an impact (but magic sales numbers aren't guaranteed either, home console Pokemon games rarely came close to the handheld mainline games in sales), but with the competition, today's market and situation and them hilariously overpricing everything, not by much. My expectation is that it will do way, way less than the Gamecube, so less than 18 million, 20 million at best (and I think even that is one heck of a stretch and requires massive price cuts). Shareholder's will be pissed and Nintendo's capital will twindle even more, so might end up being their last home console if it doesn't work out.
