Hm, honestly 5 million.
Because NSMBWii had a nice public reception due to the over popular NSMB DS.
It was the return of a classic in home console form.
Being launched back in the Gamecube days would make it a good 2D effort but yet to be proven popular.
That's why I say 5 million, and that's a really nice figure for a 2D game in the Gamecube market.