618116 on 21 July 2014
Wii U was sold about 6.1m units until march 2014, according to Nintendo, and they predict 3.6m units will be sold in this year(until march 2015). So there will be almost 10m units.
And in 2015, I think, nintendo can sell Wii U al least 5m, because there will be Zelda, Splatoon, and other Wii U games .
However, I'm sure Nintendo will introduce their new home console in 2016(maybe with a new handheld), so in my opinion, Wii U will be sold 15~17m of its lifetime, until 2015.
