One copy.
The comunity is insane huge, i swear. One Splatfest got alost 700K players online. That's huge.
The game continue to have better legs than Mario Kart 8, last time Splatoon outsold it by a huge margin.
Mario Kart 8 sold so far 7.24 million, and is still selling ( MK8 sold 1.37 million last holidays ), and lifetime will sell minimum 9 million ( most likely 10 million ). No way Splatoon beat Mario Kart since it has one year of vantage, but still, is selling more so far, it's gonna be a close battle. I predict 10 million for Mario Kart 8 and 7 million for Splatoon.