Yes, it's very possible a cat can have 1 kitten.
What is your cat doing? Does she seem to be straining or still trying to give birth, or does she seem finished, and is now laying with and caring for her kitten?
If she seems fine and doesn't seem to be trying to give birth, I'd say watch her carefully, and maybe tomorrow feel around her stomach to see if you feel anything. I'd also keep an eye on her and make sure she doesn't have more than just a little bit of discharge (a little is normal).
If you see anything that concerns you, then high tail it to a vet. But if she seems fine, and you don't feel another kitten in there, I wouldn't worry, I'd jjst be cautious and watch.