Why do dogs have so many puppies in one litter?

Has to do with the genetic makeup of the dogs, if you breed brother- sister, mom- son, daughter- father, whatever "bad traits they carry with be double fold, bad teeth, bad tail set, bad conformation, all would be super concentrated in the resulting pups, only experienced breeders, that have no problem in culling ( removing pups from a breeding gene pool by either putting down or spay/neuter) should attempt these types of breeding's. There are safe ways of breeding to be able to carry on specific desired characteristics, you need to do your research by buying books and reading about breeding before you start breeding your dogs.