It's an arbitrary number. It literally depends on your system. If your system is nutrient sterile and no food at all and no livestock, then there's nothing to clean. Rule of thumbs are bad. Consider how much the crew can even do for one, and how much they have to clean. I wouldn't have any algae right now if my skimmer pump didn't break so I'm dosing phosphate remover, have little or no nitrate, and not feeding nearly at all since I have no fish. My crew is doing a decent job cleaning up. But once it's all gone I'll have to feed them so they have something to eat. I don't think crabs do much to be honest lol