I'm bad at predicting these kind of things. But I guess I can give it a serious try.
Let's see:
Wii U: 9.5 Million
Xbox One: 10.6 Million
PS4: 15.3 Million
3DS: 54.7 Million
PS Vita: 10.1 Million
"I've Underestimated the Horse Power from Mario Kart 8, I'll Never Doubt the WiiU's Engine Again"