Yes it is unusual but a cat can have only one kitten.
Cats have two uterine horns so mommy can have kittens in each.The nursing by the baby should bring on further contractions so you want to encourage that. Also allow her to eat the afterbirth as that brings on contractions as well
It can actually take 30 hours or more for a cat to complete a four kitten(average) delivery. When she is done she will want to eat as they feel starved. She CAN have more up to 30 hours later.
A cats appetite is the best barometer of her health because if they are sick or in pain, they will not eat.
The baby happy and nursing indicates she is producing good milk.
I would not worry unless she appears upset,has pale gums, seems in pain, has an odorous discharge or another several hours goes by with obvious labor and no production of babies. Otherwise she should be fine.
Sometimes cats can develop uterine infection, the symptoms are:
pain, fever, odorous discharge, in appetite and pale gums.
As long as you do not see any of those symptoms she is ok
Please let me know how everyone is doing .I will always be here for you.
Best of luck with your blessed event