Two schools of thought.
1 big card less chance of failure but if one fails you have a problem. Usually photos can be recovered for the most part using recovery software.
Lots of little cards, more chance of one failing but you don't lose everything. Bigger risk of misplacing or losing a card than one failing.
I use one big 16g card. I have taken in the neighborhood of 500,000 digital photos in the last 10 years and never had a card fail....major knock on wood.
I used to subscribe to the lots of 4g cards theory but then I lost one, and then I lost another one. Since I went to using big cards, no problems.
Make sure you format the card in camera prior to every shoot, never move cards from one camera to another without reformatting.
Don't delete and add delete and add on a card.
Be careful loading and unloading the cards so you don't bend the pins.
I almost always delete photos in camera off the card after a shoot, this causes no problem.
Buy the best cards you can afford.