Bristlenose plecos are easy to breed...and once they get started, they breed like rabbits! I would start with 1 male and 1-2 females and have lots of caves...frequent water changes to induce spawning also helps.
You must have scattered the fry, they normally stay with the father until the yolk sacs are absorbed.
I have multiple brown BN breeders that also produce albino and calico fry. I've been dropping the brown fry into tanks with cichlids, already have too many of those.
Another way to tell which is male(when still young w no bristles) is to look at his head shape. The males head tends to be slightly longer and flatter while the females is shorter and rounder. I have also noticed males tent to have a sort of army camo with light and dark contrast. Females are more blended in color.