I think like 60 million is probably a safe rough guess.
I've heard some people claiming like 30 or 40 million, but that seems insanely conservative considering I am fairly certain that Nintendo will committ to either having the Switch be their main handheld offering OR will repackage the Switch to not include the dock or something (not sure about a micro version due to the unlikihood in Joycon size being changed, although I guess they could make the screen less wide (?)).
Assuming Nintendo is comfortable with not releasing another handheld (which seems likely at this point imo, otherwise the whole point of doing a hybrid would be useless), its hard to not see a mainline Nintendo HANDHELD with POKEMON and MONSTER HUNTER releasing on it not breaking like 60 million or so.
the real question mark to me is whether or not it will go well beyond that or not. I think a lot will depend on Nintendo potentially moneyhatting some third party developers for a few key exclusives AND Nintendo pushing their first party developments teams much harder than before (cough Nintendo hire more people, you've been mega slow at catching up to the speed that current gen games take to develop)
