I originally clicked 10-15m, thinking just over 10m units, but after looking at the GC's biggest sellers I think it would land in the 9m range, which would put it significantly ahead of the other games.
It would have done great, however, the existing install base is just as important to the games sales as the hardware it moves. The Wii has been incredibly successful (same with the PS/PS2) due to word of mouth. The lower interest in the GC would directly hurt interest in an individual game.