Well I'm guessing it will have a 5 year life cycle like GameCube.
2012 - 2.3 million
2013 - 3.2 million
Which leaves around 17 million units to sell in 3 years. I predict this for 2014, 2015, and 2016.
2014 - 5 million
2015 - 7 million
2016 - 4 million
Which adds up to 16 million.
16 + 5.5 = 21.5
So possibly if it can sell a little more than my predictions.