Available foster homes are even less. We have managed to keep the numbers at our house down to just the one litter but at this point, there are cats with litters in crates at volunteer houses. There just isn't enough room for all of them.
I know of seven cats with litters that are in the care of volunteers right now and there are many more out there that are still living under sheds, behind dumpsters and around town. I don't want to know the exact numbers ... it will just make me depressed. The unfortunate thing is that those cats who are raising their kittens out there ... well, it means the kittens will be feral.
We have a spare room so we keep them in there and keep the door closed so the other animals don't get in. Mama cat can be pretty snarly and has already attacked one other cat who came into her space (not at our house - we tried to have her sent somewhere else so we could take a younger litter).
The nice thing about fostering the kittens is that I get to have a "kitten fix" because I get to play with them each and every day (they need lots of gentle, loving handling to ensure they like people)! Then, I don't have to keep any (including mama cat); they are all adopted out between the 8 - 10 weeks marker and I get to help out in my community in a big way for a short time!
If you can provide a quiet room (perhaps a spare bedroom or a home office?) in your house for a litter of kittens I encourage you to contact your local Humane Society, SPCA or Rescue Organization.