If your cat is just sneezing and nothing else, she's probably allergic to something. Gabriel, one of my furbabies, was having several sneezing fits a day, almost every day, but no other symptoms of anything else, when I was living in another place several years ago. Since I've moved, his allergies cleared up, and I rarely ever hear him sneeze anymore - except once in awhile when the pollen count seems to be high. I don't know for sure what it was he was allergic to when I lived at the other residence, but I'm thinking it was pinetree pollen b/c there were alot of pine trees at the old place. I was also having a lot of problems myself with my allergies at the old place, which have also cleared up immensely since we've moved. I was having to take a lot of allergy meds when living there, and for Gabriel, I gave him children's Benadryl to help with his sneezing & stuffiness. It seemed to help him.
If your cat isn't having any other symptoms besides the sneezing, I would think it's b/c he's allergic to something. If he develops more severe symptoms, a trip to the vet might be in order.
I hope it's nothing serious.