I can't believe nobody has said this yet, 21 decks.
What a disappointment!!!! Earlier this week I made the decision that I was going to win this Saturday's contest. No matter what the topic, no matter how much work or what new skills I'd have to learn or how much research I'd have to do, I was going to win. Well I guess it's anybodies game now. I'd like to also say my original guess was 52, but someone already took it!