South,
I can tell you now that you are not going to be able to tell how many fish you can put in your tank based upon its dimensions and gallons of water inside the tank. You will need to spend some time looking at and thinking about what fish you would like to have, those fishes' needs, and their compatability. There are lots of considerations to make before stocking your tank, so do not jump to any conclusion. Also, when thinking about how many fish you can safely put in your tank, make sure you consider the fish's POTENTIAL size, not the size that you are buying it at.
These couple of hints do not even begin to scratch the surface of the answer to your question. I suggest that you take a look in a few fish stores near you to see what is available, or what you can order from them. BEFORE you buy however, be sure to write down the names of the fish you are interested in so that you can research them on the internet. I usually just google or use yahoo search to look up the type of fish. This way you can use multiple sources of research to verify that you are getting accurate information. Then, you can utilize forums like this one to ask the questions you still may have, and then go from there.
If your tank is brand new, I suggest finding information on how to cycle your tank so that it is ready to support fishes lives in it. Many people like to get over zealous and skimp or skip cycling their tanks. This usually ends in disaster which can last far into the future so it is best to start out with a clean slate and a properly cycled tank. Be patient, you will save yourself loads of money
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