Right now I have two decks, but I can almost make a third(just need another playest of bts). I always try to have an SP deck, a stage 2 deck, and a stage 1 deck.
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There are two reasons to have multiple decks:
1. You only have a limited time at league. If you want to use different decks throughout the day you don't want to spend time sorting them, unsleeving some cards, putting a new deck together, and shuffling a sorted out deck.
2. If you aren't at league and you need to test a deck, you can do it by yourself at home.