I would say there must need to be a certain amount of nutrients for the eggs to form. Only making them out of legs might not be enough.
As for the number of eggs on the derelict; the ship could've been a sleeper vessel, carrying hundreds of similar SJ species (we only ever saw a corridor, the transmission chamber and the egg chamber).
Then again, I think it's more likely that the derelict was a prison ship, transporting the eggs from it's home planet in a bid to get rid of them. One got free, killed the space jockey (maybe the ship's only occupant) and forced them to crash on the planet. Otherwise if it were a multiple crewed ship, I would think there would be more adults created, unless a single adult's primary mission was to generate more like itself.