i forgot one other point in my pervious post.....gas exchange also plays an important part in determining the question "how many fish?"....if you do not have alot of surface area on the top of your tank or a way to airate the tank... the envirnment in the tank will not beable to sistain as many fish.
the best way to tell how many creatures you tank will sistain is to add a few fish, after the tank is established, let the tank balance with those fish then add one more and check of all of the aspects of the tank are doing well, nitrates, ammonia, nitrites, O'2 levels, things to that nature. and keep adding until the bio load of the tank is over loaded, you will see this by any one of the aspects of the tank fluctuating. and that is when you decresse the population in the tank.