The main factor is how often you do water changes and how much water you change. If you do a 10% water change once a week then you can probably have a little over one inch of fish per gallon of water. If you did a 50% water change once a month you probably can have that number of fish but it might cause stress on the fish leading to disease. So if you do a 50% water change once a month its' a good idea to have less fish than if you were doing a 10% water change once a week.
Really its' up to you. If you have some spare time once a week then maybe the 10% change once a week would be best for you. Some people have time every day and do a small water change every day. If you go to a pet store you are bound to see an automatic water changer. Some are cheap and all you do is hook it up to a faucet and it does everything and some are more complicated.
NOTE: When doing water changes DON'T use regular tap water straight out of the faucet. If you do it could harm or kill your fish. You need to either boil water then let it cool down to room temperature of let it set for around 48 hours. The water changer that you hook to the faucet doesn't put chlorinated water into your tank. It filters the tap water elements out. You can buy filters for getting the chlorine out of water. Ask your retailer for more information.