^this

Ask Nintendo.

For the home console alone? more than 30m.
For handheld + home console? more than 70m.

If it's a console, I'd say doubling the Wii U user base would be a success. 25 Million would be enough to sell plenty of games too.

Exactly this.

Every Nintendo home console with the exception of the Wii has failed to outsell its predecessor.  The NX just turning the ship around and outselling the Wii U is going to be a tall order, but if the NX can actually double the Wii U's install base and sell 25 million consoles, I would have to consider it a success.

Of course, there is always the chance that the NX could catch lightning and become crazy popular in which case the bar for success should move up to 60-80 million.  Assuming that the NX doesn't become a pop culture symbol, however, I think 25 million is a very reasonable milestone to shoot for.

30m if it's console only, 100m if it's a handheld and console. The Wii U and 3DS are probably going to reach 75-80m combined when both are discontinued with the Wii U seen as a failure and the 3DS seen as a disappointment when compared to past handhelds. If the NX can't even come close to those numbers if it's both a handheld and console, then it's a failure.

For some people?

10 billion units at least, or else it's flop city!

In reality, anything... just not to be the whipping boy of the Vita would be a good thing. Just to keep up with the X1 or maybe move up at 3/4 of the PS4 right now, that would be fantastic work and a great comeback for Nintendo from the Wii-U failure.

I think it depends on how well software does. I wonder how Nintendo will fare. Will they regret not being able to sell two mario karts, two smash bros, or whatever franchise to their consumers.(i.e. They sold me Mariokart 7 and 8, but now will only sell us 9). Will the attachment rate be as high as the consoles, as low as the handhelds, or somewhere in between?

Depends entirely on what NX is.
If it's a console then anything better than Wii U would be a success for Nintendo, if it's a handheld then 3DS numbers would be good and if NX includes both then I thinking combining those numbers together would be good.

May be better to ask this question once we know exactly what NX is and what it's made up of.

100 mill might be a tough mountain to climb, we likely haven't seen the bottom of the portable decline. Tablets just get cheaper every year and the cheap tablets get better each year, ditto for phones. 3DS still had the benefit of the app market being in its infancy circa 2011/2012 and a kid having a $600 iPad was a rarity as it was a luxury item at first (now you can get tablets for under $100).

30 mill would be a very solid number for a home Nintendo model ... Nintendo made a lot of money with the N64.

I'm thinking maybe 40-50 mill for the portable NX and 25-30 mill for the Wii U combined with mobile revenues in the range of $700 mill-$1 billion/year would allow Nintendo to make very handsome profits. 

Unified platform is a game changer for software sales too, we have to remember that, even if NX "only" sells 70 million say between all its hardware iterations, for games like Splatoon 2 and Legend of Zelda NX that would've been looking at 15-18 million on the Wii U ... this is a huge improvement.

Third parties as well will be more apt to support a Nintendo ecosytem that has that large of a userbase, they probably wouldn't be anyone's no.1 aside from Capcom for the Monster Hunter series, but 70 mill (even 60 mill) but still good enough to get good support.