I’m just joining this conversation. I think you need to back up and listen to what everyone is trying to tell you! It’s not about digging a hole in the ground and deciding how many fish you can cram into it.
There is a “limit” to number, size, and type of fish for a very good reason. Several have already mentioned the “nitrogen cycle.” This is very important for the health and well-being of your fish. You have to have adequate BIOLOGICAL filtration that matches the needs of the fish you have. Goldfish, but especially koi produce a lot of waste that has to be converted by the biofiltration media. Yes, it’s possible to over-stock a pond and get away with it, but it’s usually only going to be your more experienced fish-keepers who have the knowledge and the desire to stay ahead of the bio-load. An over-stocked pond is a set up for disease and sick fish. Smaller ponds are harder to keep “balanced,” and we’re talking about measuring water parameters and taking appropriate action when something gets out of whack.
As a new person, your priority should be to learn the basics, start slow, progress slowly....learn. It’s exciting to start up a pond! It’s also frustrating, and at times discouraging — especially when your fish start to die “for no apparent reason.” But there is always a reason. For example, when someone recommends a particular ratio of fish, it’s because they understand what’s going to be happening to the water quality over time. You wouldn’t think it would matter, but just the growth of a few fish over a season can have a big impact on the water quality (ammonia, nitrites, etc.).
And about turtles. Nothing wrong with them. I love turtles (mine are non-aquatic, though). Turtles are very messy! This means that, especially for a smaller pond, they might be detrimental to your water quality. Just something to keep in mind.
It’s all about good water quality. And water quality doesn’t mean crystal clear water. It means water that’s balanced to reduce waste products that are harmful for the fish — adequate bio and mechanical filtration......monitoring......and adjusting.