Someone has plenty of money to spend
I'm guessing 5 would be a good number.
I have just two in a planted uk 28 gallon. I have noticed small holes in some of my more delicate leaved plants, but nothing much to worry about, i think its from when they rest on them. I supplement their diet with blanched courgette, cucumber and spirulina Algae flakes, they love the small amount of brown algea in my tank too.
http/www.petfish.net/otos_indepth.htm