You'll probably need that many once you have all of your other livestock in there. I'm kinda new to the hobby, so I might be wrong. I'm adding my clean up crew as I go. I started with about the same number that you have (with no fish) and am planning to add more as I add more fish.
The way I figure it, is that there isn't enough to eat in there for a full clean up crew and I'll starve some of them.
John