OK, the reason you keep getting different answers is because there is no answer.
That's right. People will answer the question 'how many' to try to make things easier for you, sometimes they'll give you rules of thumb, other times they'll look at your fish stocking list and make a judgment off of that, which is certainly more reliable then any rule of thumb.
If you want to figure out how many fish you can stock in your tank, here is what you can do.
First make sure all of your fish are compatible, or the whole thing is pointless anyway.
Next, measure your nitrates just before doing every water change. Note it down. What is happening with them? If they are gradually lowering or are staying even then your stocking levels are fine. If they are steadily rising each time then you have too many fish and your aquarium maintenance routine will not keep up.
This is a basic way of determining stocking levels based on water quality. Keep in mind that nitrates will rise quicker as young fish grow larger and as debris builds up in unreachable areas of the tank.
The amount of physical room is something that you must judge for yourself based on how busy the tank is, how much decoration you have, and what kind of fish you have. Water quality is a bigger issue.
Other notes:
Physical debris - if you find you can't keep up and the aquarium is steadly getting dirtier, lighten the fish load.
Algae - If you have excessive algae problems, overstocking and overfeeding are the two most common sources.
Ammonia/Nitrite - if you find that you are always detecting a presence of these elements in the water even though the tank is cycled then your filtration probably can't keep up with your fish load.
