My best guess is more than the current 3ds sales!
This is a much better product, sleaker design and more powerful than either the WiiU or 3ds, so I'm guessing 70-90 million total maybe more depending on pricing and game development.
Which will in the end be the next home to the next 3d Mario, Zelda, Mario Kart, Metroid, Pokémon, Monster Hunter, Yokai Watch, Bravely Default, Xenoblade, Splatoon, Pikmin, Fire Emblem, SmashBros and many other handheld and home console franchises united under one roof.