There are a few things to consider when figuring how many fish you can add to an aquarium. You can search fish tank stocking rules and come up with several.
It's not necessarily how big you aquarium is, but how many square inches of surface area you have. The reason you're limited on how many fish you can have in any given tank is mainly based on oxygen exchange. There has to be enough oxygen exchange between the surrounding air and the water in the tank so that the fish can breath. You have a few fish in a large tank. They will be able to breath fine. You add a few more and they breath fine as well. There will be a certain point, though, when you add fish and all together the fish in the tank need more air than is available in the water. It's not just adding fish either, the fish you already have will need more oxygen as they get bigger. At that point, you have to modify the living conditions.
You don't want to pack fish till you're at that imaginary line where they go from enough oxygen to not enough, because you need a buffer. Let's say you have a tank with the right amount of fish in it. However, you kill the good bacteria in your filter by cleaning it too well and now they aren't removing and converting the bad part of fish waste into something safe for the fish. The water quality goes down and coupled with the lower oxygen levels, your fish sicken and die.
Another way to add air is with an air stone. It pumps air directly into the water as well as when the bubbles break the surface of the tank, disturbing it and creating more surface area.
Live plants are another way to help your tank. Not only do they create oxygen in the water, but they remove toxins and waste. If you just have plastic plants and gravel then the fish rely on you 100% for the balance of the ecosystem. With plants you can actually set it up where you don't do anything to the tank, but scrape the algae off the glass and top it off. You can create a complete ecosystem. Many people find a balance between the to. The fact remains that with plants you can have more fish in the tank that without. The same is true with the air stone mentioned above.
Something else that I've never looked into, but have thought about is that lots of sites will give you a rule such as "1 inch of fish per gallon of water", but it occurs to me that just like any other animal, a fish that is more active is going to burn more oxygen that a fish that isn't. So the catfish you mentioned is probably going to find a hidey hole and rest till feeding time. The gold fish will keep cruising around the tank, so the smaller gold fish will probably use more oxygen individually than the catfish will.
So in the end it's a guessing game. The best thing to do is to start with a small amount of fish and run them for a few months. If they do well, then add a couple of more. You can also just use a test to determine if your oxygen levels are adequate. Here is a link to a test kit I googled. It provides 40 test for around $23. I don't know how well it works, never having used one, but I figure it's accurate enough. Kind of like the other test kits. You're not getting lab results, but good enough answers to keep you from harming your fish.
In fact I do suggest you just test your water on a regular basis. Many people only check things when there is a problem. From fish, to dogs, to cars, to your own body. However, if you check it regularly and keep track of it, then you have references to what is average and you'll know when your results are abnormal. Then you can treat as required. Good luck and just guessing off the top of my head, you'd be able to add several more fish, assuming a standard rectangle tank, even if you have gravel and plastic plants with just a filter. 320liters converts to roughly 85 gallons and that's a fairly large tank. I've kept more, albeit much smaller fish, in a 10 gallon with no ill effects. I was managing it closely, though.