Judging from the sales progress of similar 3D Mario games, most importantly Mario Galaxy as this sells almost exactly the same, I think it's fair to predict a steep decline in sales this year for the game. Coupled with more game releases and the age of the game, it will flatten out.
I expect no higher than 10m. My prediction is 9.6m.
This number makes the game follow Mario Galaxy, but still has it decline slightly slower, which I think it will.