one for every 5-10 gallons works nicely, but keep them in groups of 2+, so you shoul have 2-3, depending on how bad your algae is. Always add them to an established tank, they can be a lil bit sensitive. So, have your tank running for at least about 4 months.
Another idea is a hillstream loach (if your temp is 72F or under), or maybe a pitbull pleco. Keep researching and eventually you'll find one