How many cats is too many?

I have 4 cats. I had two, and no intentions of having more, but 3 years ago I found 2 kittens in a tree, and, well, you know. Just recently, I brought a stray that hung around my work home with her 3 week old kittens. I found homes for all the kittens, but now what to do with mama (who I call Cali the Calico)? I do not need, or really want, another cat. I’m already 1 cat over the limit according to the law. I am too much of an animal lover to take her to the shelter, and I cannot find a rescue that will take an adult cat. Plus, she has lived on the street her whole life, and is skittish. I can deal with her (cause I am a cat woman), but as far as someone else adopting her, I just don’t know. Since she has been at my house, she is getting better, but she still exhibits signs of stress, and she seems to want to go outside. What to do? Do I have any other options?