PM me anytime you like, I'm happy to help.
In answer to the question you asked:
The number of goldfish to purchase depends on the size of the tank they are going into. The problem with goldfish is that they grow extremely large, they do it very fast, and they are about the dirtiest fish there is.
Fancy goldfish average about 8 inches long when full grown, comets about 14 inches, and koi about 2 - 3 ft! For a goldfish to grow from 2 - 6 inches within the first 6 - 12 months should be expected. Goldfish are cold water fish, and water temp is very important for them. The warmer the water the less oxygen is in it, and goldfish use more oxygen than the average fish.
To raise goldfish, 1 - 2 fancy goldfish, about 90 gallons minimum; 1 - 2 comets, about 125 gallon minimum. Koi don't belong in an aquarium, they are more suited for a pond.
For those people who swear you can raise goldfish in smaller tanks.... they have a life span of 25 -100 yrs, depending on which type (fancy, comet, koi) and this is shortened to just a few yrs if they are kept in a tank that is too small. Any goldfish that has been in small confines for a number of years is guaranteed to have health issues, and their growth is stunted due to poor water quality. While stunting their growth, the poor water quality also slowly poisons them. Goldfish are not the easiest fish to keep!
There is no "right number" of goldfish to keep together. They do fine in any number, the problems with them are usually caused by poor water quality and not a big enough tank, feeding habits, and water temp .
Lots of water changes, proper foods (fancy goldfish need sinking food so they don't gulp air from the surface, it causes severe swim bladder problems that can lead to permanent damage and death) and keeping them in the right size environment can make for a very lengthy and meaningful relationship between pet and owner.