You want fish but how many fish should you have? There are some different theories on this but one accepted rule is 1 inch of fish per 10 gallons of water, but there are certainly many happy healthy ponds with way more fish than that. It is truly determined by your filtrations, pond plants, aeration, the overall health and design of your pond and whether your pond is conducive to fish breeding. Some happy ponds are built where the fish have baby koi like crazy and some ponds have the opposite problem where no matter what, even if the fish breed the fry get eaten.
Overpopulation of your pond with fast breeding fish or just over stocking throws the ecosystem out of balance and leads to fish-health problems. Not having enough fish can mean nutrients in your pond will not be fully absorbed and recycled as they should. Talk to your pond pro or fish retailer in your local area to get their opinion. You will probably find different answers from different people, and certainly the internet will have its say on the topic. On existing ponds, if you are having fish health issues due to overload, consider upgrading your filtration and aeration, enlarging the pond or maybe build a second pond and definitely avoid over feeding the fish. With a new pond installation if you want a lot of fish, your best bet is to increase the pond size and filtration from the beginning. It is always recommended to start with only a few fish until the ecosystem balances itself out.