I'm going to guess 60-70 million with the caviot being Nintendo does actually fully commit to it, rather than release another handheld system. If in 2 years somehow something like a '4DS' releases then I would cut my prediction in half
but if Nintendo genuinely does commit to dedicating both their handheld wing and home console wing (in terms of production dev teams) to the Switch, then I see it most likely matching the 3DS in sales. Price points will drop at some point
again though, this is under the assumption Nintendo really is merging their markets. If a 3DS sequel system comes out then that would damage the Switch because there are always going to be some gamers who would prefer a a handheld specific system for a cheaper price point.
The reality is Nintendo at this point would probably be wise to end the 3DS in a year or so and fully commit on the Switch. IF they give consumers options and divide the fanbase again, the Switch would be cannibalized because they would essentially have two wings that both share some of the handheld market
I see no reason it won't sell fantastically if Nintendo is going full force on it. And by that I mean exclusive Pokemon games, exclusive deals on Yokai Watch, exclusive deals on Monster Hunter with Capcom (an automatic assumption if the Switch is to be their future primary handheld device), and then all of the regular series for Nintendo's home consoles
those suggesting like 30 million are being a bit unrealistic if the Switch does end up being Nintendo's (basically) only platform for the next few years. In the end I think its less of the hardware that would bring people in as much as there are enough franchise fans to loop in to make pretty high hardware sales
a new Mario Kart + new 3D Mario + Pokemon + Monster Hunter? if the dedication of titles is there and focused specifically on the Switch I think things will go very well. a unified library is key and arguably the #1 selling point of a system like the Switch IMO. Conceptually it is what the system suggests anyway, going from TV gaming to portable gaming in a moments notice. Which is one big reason that I am dismayed Nintendo hasn't hinted something about Pokemon on the Switch immediately. seems like a no brainer way to suggest to consumers that "hey, we're bringing the handheld market hard with the Switch"