The gestation period is around 65 days from conception. If you did not plan the breeding, it would be impossible to know when to expect delivery.
Cats usually don't need a lot of help to have the kittens. If she is an outside cat and you worry about her hiding the kittens, you can confine her in a garage or other area that she's used to until the delivery date.
You will continue to get more and more kittens unless you spay the females.
There are low cost spay and neuter programs in almost every city that you can take advantage of.
Let me know if you still have questions.
