Each deck has its own maximum spell copies, which may be different for main school and others, and a side-deck for treasure cards. If you look at the item specs on character pages or in the bazaar you can see the details.
Generally count goes up with level, and is higher for your main school. At level 35 you can get 6 of main and 4 of others. The specific deck will depend on school. At level 60 there are a couple decks available with 6 of any spell.
