If you pay your card off each month, cool. In my household, we have 4 credit cards for various reasons. However, both my wife and I only keep one on our person when we're out and about. There's absolutely no reason to carry more than one.
If you are going to use credit cards, make sure that you do the following:
1) Find a card that has no annual fee. While you might not pay finance charges if you pay your ENTIRE balance month to month, annual fees can add up if you don't pay attention. A $75 annual fee may very well be worth more than the 1 free hotel stay you earn a year or 1 free airline ticket you get every third year through credit card points (unless of course you charge an obscene amount of money):
Example:
Average family may charge $1000 a month. $1000 = 2000 points. A hotel stay = 25,000 points. That equals just over 12 months of credit card charges to get a free hotel. With airline tickets costing 60,000-75,000 points, you can see how things don't look so good in the long run.
2) Call the company periodically and renegotiate your interest rate. Most companies are willing to lower your rate by a bit, particularly if you have a long history with them, and you have a lower balances that can be easily transferred. They don't want to lose your business.
3) Don't purchase "loss of income" insurance (or whatever you card company calls it). You shouldn't be carrying a balance any way, so the need to have them pay the minimum payment doesn't apply, right? Besides, if you read the fine print, there are often waiting periods or other conditions/terms that make the programs a lot less "valuable" than they might seem.