Well, there are lots of ways to determine how many fish per gallon in you pond. You can have more fish, for example, if your filtration system is really good. It also depends on how deep, and wide and such.
Here are a few general rules of thumb:
One 6" fish for every hundred gallons of water OR
1" of fish per 10 gallons of water OR
Total gallons of pond divided by 200 = Total 6" fish in pond