Mating process in cats?

I'm taking care of a stray female cat for 4 months now. It's totally okay for me for her to have litters and I've already taken care of their bed space and adoption, now, a tom is currently mating her, actually, the tom has mated with her twice, and there are no other toms in the village, and all the male cats are still kittens so there is only one possible tom-mate. My question is for how long will that tom keep on mating with my cat till my cat gets pregnant? Or was she already impregnated the first time the tom mated with her. I've heard of queens being sired by several toms, so that means several mates, but what if there is only one tom? How many times will they mate? And what does it mean when this tom keeps following my cat? Always following her WHEREVER she goes. I've searched the internet and it only tells instances like several toms and gestation periods.