I think the only way to know is to get an x-ray and count spines or skulls. Being her first litter they commonly have less, so 2-4 kittens I would guess. But 6-8 kittens or more wouldn't be out of the question either. There are a lot of variables though, like size of the cat, as well as age, nutrition during pregnancy, health, and environment (calm or stressful).
Some vets can tell by palpating her belly. I'm told they can feel the skulls of the little ones inside. Tried a few times myself on neighbors' cats over the years to see if I could do it, but never got the number right. Maybe one of the local vets around you can do it--it has to be cheaper than X-rays.
My cats always look like bowling balls on toothpicks! They couldn't even lay on their sides. She is not that large, and being the first litter, I would say no more than four. I am betting on three.