after the Pokemon league? (I think you get more space after you defeat the champion)
One box can hold 30 Pokémon and you'll get 24 boxes. 30 x 24 = 720.
In Black and White 2, you start off with 8 pc boxes (8 30=240). Of course you are not limited to 8 boxes, for once at least one Pokemon is places in each of the 8 boxes, the storage capacity is increased to 16 (16 30=480). Once at least one Pokemon is placed on each of the 16 pc bpxes, the maximum capacity will be 24 (24 30=720).
You don't get extra space* after defeating the Elite 4, but you do get extra wallpapers.
Hope this helps c:
As Jofly said 30 per box but you can get 24boxes.
30 x 24 = 720
720 Pokemon you can store