How do you know you want to have children?

I'm 22, and my boyfriend and I have been together for 3 years. We've talked very little about if we want a family. I always thought in my mind that one day, I would have an epiphany, and say "i want 3 kids!" But I am still undecided. I feel that kids are something to pass on your stories/legacy (?) and I definitely feel I have much to teach to someone. Could this be a reason to have children? Or will that day still come when I finally know 100%? Any advice from parents/non parents? Just wondering....