An old rule of thumb is an inch of fish for every gal
However, that is pretty arbitrary as there are variables involved.
How big is the water column..... how much water is eliminated with big rocks etc ?
What kind of fish are they? Some are much dirtier than others..
How much free time do you have to do maintenance like water changes etc?
So, one must do some research on the fish they want to get.
Also, add more fish a little at a time and keep a close watch on the Nitrates.....keeping in mind that Nitrates are not nearly as toxic to fish as Ammonia and Nitrites are......
and that Nitrates are necessary for plant growth.
Most all tanks are a bit different and what may be overstocking in one is not in another
So only a given tank owner can really assess if their tank is overstocked or not .