Once your cat starts pushing, she should have the first kitten within 30 minutes. If not, or if you see any frank bleeding, take her to the vet. A cat's uterus has two sides to it, so it's normal to produce 1-3 kittens, take a break, then produce the rest. Keep her hydrated throughout the labor and let her eat the sacs/placentas. It gives her nourishment.
Please spay her just as soon as the kittens are weaned, and encourage the people who take these kittens to have them spayed/neutered ASAP, before they give birth. There are literally millions of unwanted cats in this country, we don't need more.