If you need to generate a random number within your app, you have to push aside Objective-C, as there is no class with a built-in random number generator. The alternative is to use C, among the functions available are: rand(), srand(), random(), srandom() and arc4random().
arc4random() tends to be preferred as it does not require seeding, the function automatically initializes itself.
// Get random value between 0 and 99
int x = arc4random() % 100;
// Get random number between 500 and 1000
int y = (arc4random() % 501) + 500;
Update: thanks Dave Verwer and Kevin Conner for pointing out the advantages of arc4random_uniform.
arc4random_uniform() will return a uniformly distributed random number less than upper_bound.