I just recently purchased some Oto for my 55 gallon to clean up the algae. They're doing a pretty good job so far, although there's still a lot of algae on the sand.
Ottocinclus need to be kept in groups of at least 5. 8 is a highly recommended number.
I didn't have any algae eating fish in my 55 gallon aquarium so I bought 6 to add to it, and supplement with sinking algae wafers when I feel the need to.
I would buy at least 5 if I were you.