i guess 2mio till end 2015.
Its a very nintendo and wii u typical game. Also there is a big lack of new games at the moment for the wii u (half year 2015 was horrible), so i guess many wii u owner are looking for something new to play.
Also keep in mind how bayonetta 2 or hyrule warriors sold on wii u. Both are very untypical games for wii u. Hyrule got still 1m and bayonetta 600K (?).
+ it is also cheap in europe (just 35€).
I would definitly expect much more then bayonetta or hyrule warriors in terms of sales.
Also checking amazons gaming charts, splatoon has a good spot (better as witcher 3 for xbox on in my country).