Yeah, a female guppy can spawn when she's only a few months old or about that, and she can have several spawns from one mating. You could potentially avoid that if you looked carefully at the guppies in the pet store. If the females are large and gravid, they're pregnant already. (they look like bloated) Same rule applies to any livebearer, including mollies and platies and swordtails. I would start with one male and three females or four females and see how the girls react to them before getting any more. Or, if you get one male and four females, you won't need to get anymore, they'll do all the work for you.