I heard suggestions of one per 10 gallons, but two would be better so he has a buddy. Make sure you suppliment with algae chips when you don't have a lot of algae.
My ottocinclus are not bottom feeders. I never see them scavenging on the bottom.
Mine eat brown algae, algae wafers, dead plant matter and shrimp pellets and mushy cucumber. Once the food is gone they are back to hanging on the side of the tank.
Yah, otos aren't "bottom feeders" in the sense that cories are. They mostly stick onto the glass or plants and clean those off. They really only eat slime algae and other mushy substances.
I'd get 3 if you have the space. They prefer groups. But make sure that you'll be willing to buy them algae wafers and cucumbers/zucchini once in a while...