Based on your measurements, your pond is around 800 gallons. If you don't put a full fledged filter system on the tank, you should at least put in a fountain or waterfall and then you could make a DIY pre-filter for the pump which would also provide mechanical and biological filtration for the pond... as well as airation from the fountain or waterfall. If you go to my blog (link in my sig) and then look on the right side, you will see my links to DIY Pond Filter. This is a very easy filter system to make for a small pond and costs very little... usually around $10.00 US. Of course, the fountain or waterfall kit would cost much more but you might be able to find a used one at a garage sale or classified section.
Adding some filtration/airation will greatly increase the number of goldfish you can have. Without any filtration, you would be limited to just a couple of fish. With limited filtration, you could probably go with one per hundred gallons. Do not get any Koi though.. they get much too large for your pond. Some comets and shubunkins would be best as commons also get larger.