Remembering that comet goldfish can live for 20-30 years, and reach over 12 inches long, let's go back to the old fish stocking rules of thumb. For most freshwater fish, 1 gallon per linear inch of fish. For goldfish, 2 gallons per inch. This is, of course, assuming a very robust filtration system. So, a 12 inch adult comet will need 36 gallons each.
For such a small pond, I really would not exceed 5 comets. They grow large, eat a lot, and make a lot of waste. Luckily, they are neither as large nor as destructive as koi, and most aquatic plants will be safe with them.