I think about 60-70M, if Switch is really the continuation of both Wii U and 3DS. A huge hit in Japan is basically guaranteed, and with the developers of the 3DS flocking over to Switch, there will be no drought this time around even if the major western publishers would decide to jump ship... again.
For those who are asking "why not more then?", 3 reasons:
First one: The price. Switch will be way more expensive than a 3DS no matter what during the latter's lifetime, and not much cheaper than Playstation or Xbox at least early on. This will hamper sales early on, and delay the move from 3DS to Swich by the developers.
Second: A shrinking console market overall. While not shrinking as fast anymore as in the past, it's still on a downward trend. With the PS4/Pro already having a big chunk of the market and Project Scorpio probably taking a good chunk too, there is just not enough left for Nintendo to get bigger sales.
Third: Probably short lifespan due to the power limitations of a hybrid console. Though a pure mobile version could continue well into the future, the home console part will be outmatched as soon as no games will be made for the "base" PS4 and Xbox ONE/S. Which would mean creating Assets just for that console, which is probably too expensive for the big publishers by that time.