They get hit by cars or starve to death. Cats and kittens usually survive longer than dogs when set free because people will feed them. While this is a noble gesture, these animals need to be spayed and neutered. If you are the person feeding these cats and kittens, try to capture them and get them spayed or neutered. Each female cat can have three litters a year.
Most litters are between four and six kittens.
Depending on how many females are in the litter, at about the age of four months, each female kitten can start its own cycle, etc. You do the multiplying. Don't delay, especially with cats. Time is of the essence. If you don't have the money for the clinic, call your local shelters and they can probably refer you to someone who can help you.