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.
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.