Two Inches of Koi per Ten Gallons of Water
This "two per ten" stocking density is 100% "do-able" however you will have to be constantly vigilant about your pond water quality water quality, and the stability of your pond pH and you should be aware that accumulation of organics, background pollution and bacterial overgrowth will be constantly troubling.
Excellent, large filtration and careful attention to water changes will prevent health problems, but fish will still grow more slowly, at the two per ten level.
In my [ELJ] fishkeeping career, I have seldom kept less than two inches of fish in ten gallons of water. I have, over time, paid dearly for that "high intensity" stocking level.
Here's a good [better] idea about stocking.
"One inch of fish per ten gallons of pond water will do well if adequately filtrated and small, regular pond water changes are done."