It goes without saying that Nintendo needs to sell more hardware than the Wii U, but the deeper question is to what end?
If Nintendo was like any other company, then it'd simply be to make more money. Not that Nintendo will say "no" to more money, finances are a HUGE driving force behind their thinking.
Instead, I propose that if Nintendo wants to succeed in the role that some proscribe to them, that of a game maker who is influential through innovation, uniqueness, and high quality, then a larger market presence is naturally required so that they can create conditions for that innovation not just for themselves, but for other game makers, and so that they can reach an audience who will be inspired and carry on those values in their own work elsewhere.
Now the problem would be defining at what level they might achieve that. "Somewhere between the bottom console and total world domination" is simply too broad a range to be useful. The Wii U was obviously too small relative to its market and environment, Nintendo's ability to exercise its values are constrained under those numbers. Likely the GameCube's userbase and demographic reach is likely too small as well.
What about the N64, and it's 33 million units shipped versus the PlayStation's 120 million units shipped? If the Wii U enjoyed a similar marketshare in this generation it would have sold.... about 14 million units... so maybe not? Yet the N64 and its software library was highly influential in so many ways despite the userbase ratio it experienced.
Perhaps there's a magic number at some point, like 35 million consoles, at which the user base is large enough in gross terms that marketshare might be irrelevant. That's a low mark to hit fiscally, obviously Nintendo's money counters want to sell more than that and return to Wii-like levels of profitability.
But for the sake of argument, IF Nintendo could make oodles of money at 35 million hardware units sold, AND garnered visible-if-not-complete third-party support, would 35 million hardware units sold give it enough of a platform to freely exercise their ambitions, make the games they want, and inspire a whole new generation of players?
To be clear, its likely that anyone at Nintendo who proposed a plan to sell only 35 million hardware units over the life of the NX would be fired on the spot. But aside from the strictures of ambition, would that number be enough to achieve the specific goals we proscribe to them, and if not, what? Keep in mind that the current gen of home consoles (Wii U, PS4, XBox One) have only sold around 72 million hardware units cumulatively up to this point.