Define "deck". I have 3 decks at any given time, but I carry around some of my deck making materials to tournaments, in case I'm using Kingdra/Scizor, and everyone there is using Gigas.
I've got enough of the 'key' cards (Rare Candy, Claydol, etc...) to make four decks at a time. I just don't see the point in having more than one made at a time, though, considering only one can be played at a tournament.
I have about 20 decks at one time give or take a couple. I have around 6 decks that are up to speed for the current format. These decks can definately handle their own in tournaments.