Yeah. 30M seems about right!

It needs to show notable growth over Wii U, so 20m up is decent if the console itself is profitable (ala Gamecube).

The question isn't simple though, we really ought to now what the system is, it's entry price. It's profit margin and intended life cycle

The N64 wasn't really a success either. It did really well in the USA, but it was a failure in europe and japan.

I still think 30 millions wouldn't cut it. We're not talking about an "ok results", we're talking about "success".  

Here are the home consoles from Nintendo that are considered success:

-Wii 100 million

-NES 60 millions

-Snes 50 millions

You can't go lower than 40 millions lifetime.

According to some posters here the PS4 would be a smashing success, even if it stopped selling today? I´d assume most people would call that an epic failure!
Anyways for NX to be a success I say it can´t be "last place" in this three legged race (or at least make it real close like last gens 2nd+3rd). In numbers I´d guess that means 60m+ Homeconsoles sold. If it is a Hybrid (doubt it) then 50m+65m(handheld market definitely has shrunk but Nintendo now has a monopoly).

For start it needs pass at least GC numbers, but I would said more than 30m.

40 million (if you're talking one model, that being a home console) is probably asking for a lot, to get there Nintendo probably needs a couple of new breakout hit IP. N64 basically sold the same as the SNES in North America and Europe, it's Japan that basically accounted for the difference and that was strictly on the dumb decision to not inlcude CD-ROM as some part of the N64. If they had CD it would have outsold the SNES, probably easily because it would've had JRPGs and many other types of Japanese games the Super Famicom had.

Yes I'm talking about one model, home console. If the NX is a hybride handheld/home, the numbers needs to be a lot more than that. It needs to sell something upward of a 120 million to be a success (the 3DS/wiiU are peaking right now at 82 millions combined, and probably will hit 90/95 until they are discontinued, so the  NX needs to sell more than that,  otherwise it will be a step back, and the last thing Nintendo wants, is another step back) 

As for the N64 it wasn't a success in japan and europe, and overall, not a success. It was half-successful. But we're not talking about what the NX needs to be a "half-success"', we are talking about a Success.

30 millions is not a success in this market, it's not a success for Nintendo either. 30 millions would be a good start but a disappointment.

I say 40

I voted 30-40m.

If the homeconsole edition of the NX, sells over 30m, Id consider that a success.
Nintendo has had this downwards going trend of selling less and less consoles.
If they can turn it around from the Wii Us 13m -> 30m+ with NX, that would be a success.

30M+ would already exclude defining it a flop
40M+ should be enough to define it at least a moderate success

20M+, but les than 25M would be a small flop, but it should still be enough to keep Ninty afloat comfortably and allow it to try again.