I don't trying to put rules of thumb on CUC's. Mainly because every tank is different and therefore has different needs.
What I normally tell my customers, is to add the CUC a few at a time. You want just enough to keep up with the chores and no more. If you add too many, and they get the tank spotless, then animals start starving and you'll lose money.
I'd say to add 4-5 of different critters until there is just enough to keep the tank clean.