There are a lot of really big numbers out there claiming that a pair of feral cats could be responsible for producing 420,000 offspring.
and this site has a chart showing production over 10 years
figuring 2 kittens per litter and a 10 year breeding life.
But feral cats don't usually really live 10 years, few feral kittens survive unless there is a lot of food and safe housing and territory resources available. Here in the northern US feral cats have lots shorter lives than in a warmer climate. Cars as well as weather account for feral cat death plus we have predators where I live that are happy to add cat to their diet.
If they are being fed they last longer than if they are relying on local resources.
Some consolidated info here
But as uncontrolled breeding goes on in many areas and we are not hip deep in cats I'd say the projection numbers are way higher than the reality.
In general there will only be as many cats as the available food and resource supply will allow the same as with any other population.
Lets say you have an area that can support 60 feral cats. They live and reproduce on their own. When you come back to the area there will likely still be 60 feral cats but not the same ones as only the strongest, smartest, healthiest will have survived to continue breeding.
If someone comes in and feeds them the population may go up.
If someone traps alters and releases them then the altered cats have a better chance of surviving than those cats that waste resources on breeding. So you can set up a stable feral population using trap neuter and release. Newcomers will be pushed out by those already using the resources.
If the population goes up a disease is apt to go through the colony dropping the population again or fewer kittens will survive as there won't be enough food for them and stronger cats may kill them.
Say its two kittens per litter one male and one female - then check that chart I listed if you want the theory rather than the reality.
Hope this helps you!