I just recently bought some, I think it was 11 or 12. I used reef town, in about 2 days a single rock was cleared, but i would guess for my entire 120 gallon to get cleaned, would take a few months. Otherwise they seem to work and dont let the aiptasia breed like aiptasia-x and others do. This is an expensive route though as you need to buy them in little herds, enough to start breeding and certain fish will eat them, so you need to account for that too.
I have two wrasse a ornate leopard and a dusky wrasse, luckily they haven't seen them as they come out at night and my wrasse sleep in the sand at night, but wrasse will eat them.
reef town link