I don't know who said to allow one goldfish per 10 gals of water, but, when I look at the way my goldfish are growing, I can't help thinking that he or she may be right.
I have 13 goldfish and 2 weatherfish in a 55 gal. tank. I started it early this spring with 2 short tailed feeder fish (one was rescued from a turtle tank), 3 fancys and 9 comets. One of the comets died shortly after purchase.
All the others are sturdy and healthy fish. Until a few days ago I also kept 2 African Clawed Frogs in the tank but I think the goldfish were starting to get all the food, so I took them out.
They do eat a lot and would eat even more if I gave it to them. One problem I seem to have is that they are very active and tend to keep the water churned up past the ability of my filter to clean it. Even daily siphoning does not really help because the wastes are broken up before I can get to them. I will have to get a still larger filter or add another to keep up, especially since the problem is getting worse as they grow. The tank is healthy though and I've never had a disease problem in it (parasites, yes).
They are beautiful fish, pets, really. They enjoy each others company and I would hate to see anyone keep just one in a 10 gal tank; I hope no one gets that idea.