I carry anywhere from 3 to 5, plus whatever other decks I have under construction at home (6 right now... Construction takes more time and effort than actually having the deck, I'll build a deck for weeks and play it for three days before deconstruction.) Right now, I have my Clear the Land/Prosperity combo deck, a 5cG thing, a red Light 'em up Like a Torch deck for duels, a Stampy varient, and a Fish deck that kicks butt.
I have different decks on me depending on a variety of factors, my mood, the current metagame people are playing, etc... If I'm mad, I play Wildfire and get ganged up on after the first game. And when I realize that the Freshmen at my school have never seen the Greater Good/Alter of Dementia combo at work, I bring that in and get it 'informally banned,' in the words of Duel because it wins too much. If I'm really tired, I'll play beatdown and not think, if I got plenty of sleep (yeah, right, when was the last time THAT happened?), I'll play D5C and make each game take more than two hours. Etc, etc, etc.
Btw, I'm obsessed with 5 color lands and basic set painlands. If anyone has any for trade...
"What do you mean, 'informally banned?'"