I've usually found with my common & comet goldfish they grow 1inch every 3-4months, growing more when they are small and slowing down in growth as they become large. I agree with Black Angels estimate of 1 goldfish per 100gallons- if you had REALLY good filtration and a large pond you could probably stretch it to 1 goldfish per 50gallons, however in ponds goldfish can breed like rabbits (they usually spawn in spring) and so i would always recommend leaving some unused gallons in the pond to allow for occurrances like fry. I have 16 goldfish in my pond and they have had so many tiny fry i can't even count them all!