Do you mean the ones with yellow tentacles in the sand or crevices? These are cirratulids. As long as they're alive & healthy they'll keep reproducing. Problem is, if they're putting out eggs & sperm into the water for fertilization & development your skimmer or other animals are likely to wipe any juveniles that result. On the other hand, many cirratulids reproduce asexually by splitting into pieces; each piece then regenerates the necessary missing bits. If your worms belong to one of these species then you'll soon have a large population of them.
Or do you mean terebellids - the ones with white tentacles? Nearly all of them broadcast sperm & eggs so these are less likely to successfully reproduce.