It really depends on your aquarium, the equipment within it and your maintenance regime
What I mean by that is when you bought the aquarium did it come with all the kit (filter, lights & heater)?
If it came with the kit included, then more often than not that kit will be basic and close to being maxed out for the size of aquarium....that in turn limits your stock availability and choices.
If you replace the filter with one that is larger in capacity and flow rate, then you might be fine to overstock, but even with an oversized filter you will still be limited on stock by virtue of the space within the aquarium...ie...you won't be able to have larger species in a relatively small space.....that is common sense.
Your maintence (husbandry) is also of vital importance regardless as to whether you upgrade the filter or not and your stock levels.
Weekly minimum water changes of 45-50%, rinse out media in the old removed water and changing media when it literally starts to fall apart, hoovering the substrate, not overfeeding, maintaining healthy water chemistry is essential when overstocked (more fish = more waste products = more chance of deadly spikes = more potential for fish death)
I can sense your frustration but there are many aspects to keeping fish that people coming into the pastime do not realise or understand, often not realising a problem exists until its too late.
So when asked what your maintenance (husbandry) is and asked about the filtration you are using it can be extremely helpful to those of us trying to advise you on the best course of action. We are not being critical, we just need to know what your aquarium is like, what you are doing to maintain it and the stocking you would like against what would be appropriate for that aquarium.