Hi and welcome to TCS. The last pregnant cat I was around, had 3 kittens - the 2nd and 3rd one came about 24 hours after the first. It was her first (and last) litter. It seems that is fairly common. In some cases, I have heard the births can happen weeks apart because the mama was actually impregnated at two different times, although I think this is much less frequent.
As long as your mama cat is acting fine, and eating/drinking/nursing, I'd say just keep an eye on her. If she shows signs of distress unrelated to the act of delivery, then you know to call the vet.
Agree with above.
Also, its common the owner feels the enlarged / swollen uterus, and thinks its one kitten left... One owner even swore she felt movements, but it was just some gases...
As advices above; do observe closely, but stay calm as long as she is OK and no other alarming signs... It will usually resolve itself this way or another: either more kittens, or no more kittens.