Fostering 30 cats is NOT the same as having 30 pet cats.
The organisation who has arranged the fostering (e.g., Cats Protection or a cat and dog shelter) are usually responsible for all care costs such as vet bills, food, litter, medications, etc.
The cats will have already been neutered before going into fostering or, if too young or sick etc, will be neutered shortly after.
They have to be kept seperate from your own pets, if you have any, and usually aren't allowed outside except in an enclosure such as a cattery.
They won't let you take more cats than you can safely house and care for. If this woman has 30 then she obviously has room for them and, as PP have said, some of that number are likely litters of kittens.
She has 30 "at the minute". Foster cats don't usually stay forever, they stay anything from a few days to a few weeks to a few months before they're ready to be rehomed. 30 will be a transient number that frequently changes.
DH's aunt fosters cats and her house does not stink, it is not full of cats, she has had no surprise litters, and they're all fed, wormed, flea treated, cleaned out, and cared for. She has a cattery in her outhouse.