start with 4 or 5. If they're not keeping up then add another.
They eat detritus, diatoms, hair algae, and cyanobacter. They're a good all-around clean up critter. They will need at least 1/2" of sand to hide in.
i'm still sort of new to this i'v had smaller tanks before but i'm tryin to learn alot and tackle a smaller reef tank i actually just got a 28 gal bow front i returned my 15 it had the eclipse hood and i figure i'll have to replace the lighting and it will be difficult to manage all the hang ons and such sooo i just went with a bare aquarium