If it's portable, the floor is 50 million no matter how bad the games or the hardware itself is, even if the machine was more expensive than a PS5 there is no way for it to sell less than 50 million.
If it's not portable, Wii U level of sales are not out of question if the hardware and software are bad. Wii U was killed quickly, so 20 million assuring Nintendo give it at least 5 years of proper support