Many more will end up at the shelter. These six females will go on to have their 22 surviving kittens each. Realistically, over 12 years, one unspayed female, with all her unspayed female offspring, reasonably can be expected to be responsible for over 3200 kittens if there is no human intervention. Some continue to advocate the trap and kill eradication approach. However, if eradication programs really worked, we wouldn't be faced with so many stray cats and their offspring at the shelters.
Cats are territorial.
They don't allow other cats into their territory to steal their food. Altered cats will stand their ground and guard their food source, will not have kittens, and will die in a few years. Once the offspring of these feral cats are over about six months of age, it is nearly impossible to socialize them to the degree necessary to be placed as house pets. An unsocialized cat is an unadoptable cat. The Cities and Counties pay for the handling of these stray cats and their offspring.