For arguments sake let's say it sells 5.5 million by the end of the year (Actual number will probably be just under) I think realistically it could sell 7 million units in 2014 putting the total at 12.5 million. I suppose if everything goes off wrong the worst case scenario would be around 9.5-10 million total and if everything goes better than expected we might see a total of 14.5-15 million. Although software sells hardware and 2014 has some awesome Wii U games it's hard to convince people to buy a Wii U when for $100 more you can get a PS4.
Predictions! By the end of 2014... PS4: 15 million +, Wii U: 12 million, XOne: 9.5 million!