How many litters are safe for a cat to have in it's lifetime?

I had a cat a few months ago and she had had 2 litters with her previous owner, she is 5 years old and fit and healthy, since i have had her she has had one litter which were born a couple of days ago, would it be safe for her to have any more in the future? as we would like her to have a couple more and then keep one or two from the last litter so that when she passes when she's old we have got another cat. We have had no trouble re homing the kittens so it wont be a case of them being in shelter infact all the kittens she's have have being reserved for sale before they have even being born.