How long has the tank been set up for?
What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
-----------------------
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate new water before adding it to the tank?
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you feed the fish?
What do you feed the fish?
-----------------------
These factors all go into determining how many fish can be kept in an aquarium. A big tank with a few small fish might be fine if you only do a water change once a month or if they only get fed a small amount. If you have bigger fish and feed heavily, you might be overstocked.
You need to monitor the nitrate levels and if they don't change between water changes, and the fish have space to move without running into each other, then it's probably not overstocked.