How many bases are used in a game?

Bases are typically swapped out at the end of the 3rd and 6th innings.  Some get cleaned and will reappear in future games, others get pulled out and made available in Stadium shops and to game used memorabilia collectors.

Are bases from the current season?

Bases may be from the current season but also could be from the past several seasons. 

If the base is dirty will it stain my wall?

Surprisingly, no. We remove any loose dirt particles and the dirt you see on the base is very much ingrained in the vinyl material of the base itself.