That may be hard to answer. I can only give my experience. I have 125 gallon tank that had an asterina EXPLOSION! I manually removed well over a hundred within a few days. I put one small harlequin shrimp in the tank shortly after, and he made an impact almost immediately. He has been keeping them in check for the past 6 months (and he's grown like crazy). I still see several asterinas on the glass every morning before the lights come on, but they're not a problem like they were before. I'd love to have another harlequin shrimp, but I'm sure there would not be enough for 2 to eat. If I ever stop seeing the starfish, it'll be time to re-home Harley.
So to answer your question, I really don't have any good idea, but it's a lot. Hope this helps.
