55 million.
I believe next gen total sales will be smaller than this gen and I believe Nintendo will lose significant marketshare and the Wii U will have a shorter life on the market than PS4 and X720.
PS4 - 100 mill in 8 years until the PS5 comes out
X720 - 90 mill in 8 years
Wii U - 55 mill in 6 years