"rule of thumb" is dangerous!! If you are looking into keeping a certain bottom feeder you should consider if it is a schooling fish, and get at least 6 to create a school. If you don't have a bottom feeder in mind, you might consider what you have for other fish and what bottom feeder would fit into the tank (with the bio load in mind).
Bottom feeders help because they eat the non-poo debris, and they stir up the poo and help it along to the filter (perhaps). So you want to keep that in mind when deciding how many. As far as it being a rule of thumb, then you get into mythy-waters, and myths can get you into trouble (ie, inch gallon rule).
Anyway that is my best advice.