One is enough! I use it for emergencies.
When I was fresh out of college, I used to receive four to five letters a day from credit card companies begging me to use their. Like an idiot, I applied for and received four different cards (AMEX, Discover, MC, Visa). What a mess. Sure, I had "buying power," but I also had debt that wouldn't be paid off in a hundred years if I only sent in the minimum payments.
I hope I'm wiser for simplifying my credit life.