Almost all rackets today are made from some combination of graphite and other composite materials. These rackets are made of thousands of very small, very stiff fibers. Each time you hit the ball, some of these fibers break. When you break a string, the racket will briefly change shape, breaking even more fibers. This will happen sooner, if you play very often, hit the ball with a lot of force or break a lot of strings.
Many pro players carry up to seven replacements in their bags.
For recreational players, whose rackets typically aren't strung as tightly as the pros, two or three replacements should be sufficient. Don't get frustrated, get a grip. Changing your grip will help you retain your hold. Tennis isn't a contact sport, but painful blisters and minor cuts can occur during the course of the match, so Band-Aids can provide instant treatment. To avoid dehydration, drink plenty of water before, during and after a match.