A female starfish can lay up to 65 million eggs! That's a lot of baby starfish in the sea.
A male and a female starfish will reproduce in quite an usual way, they carry their reproductive systems on the ends of their legs, and when they find a suitable partner, they will both release at the same time.
It is for this reason that there are usually a lot of baby starfish.
Another reason for the large amount of offspring is that often a large group of starfish all breed at the same time, so there can be many reproductive systems floating about and all mixing and mating together.
The babies are only about 1mm when they hatch, only just visible to the human eye, so many of them die before they reach adulthood.
Biologically, this is why the starfish need to lay so many eggs... so they can guarantee that at least some will grow to be mature starfish.