We rarely contend over goldfish - the issue comes into play when people apply the "inch of fish per gallon" to koi in a pond. 30 inches of goldfish is a whole different animal than 30 inches of koi. I would also mention that "a goldfish" can be an inch long or 8 inches long - big difference.
Here's a link to some information regarding the amount of waste that fish produce at different sizes that I find very helpful (because ultimately that's what we have to assess - "too many fish" means "too much waste produced') :
This chart also lists koi weight by weight and how much ammonia and waste they produce. Koi get heavier as they grow and produce more ammonia and waste.
russellwatergardens.com
Also, not to stray too far from
@LondonPond 's question - this pond is 30 gallons total with 10 fish. Again, 10 tiny fish may be fine. Give them a year to grow and it's a whole other story.