I think 100m, 80m minimum.
This is assuming that there is a Switch pocket released at the arrival of the first Switch exclusive pokemon game.
The platform in its current form has infinitely more appeal amongst people that I know over the Wii U. No one I know owns a Wii U (I live in the UK) but a handful have a 3DS, the Switch right now has more demand in my friends group than either of those systems. In terms of software its already kicking both Wii U and 3DS' first year lineup wise and I don't expect that to stop any time soon. I think it will have a better chance at securing adult gamers than the 3DS did... at the same time I also heard one parent refer to it as the "Wii 2" to their child :p
I could see the system in its current form selling 50m but I think the completely portable $199 revision that will arrive sometime down the road could double that number. The amount of double dipping will be huge (especially in Japan)