It depends on the fish that are supposed to live in the tank. 50 neons in a planted 55 are ok but not in a 20. 1 oscar can live in a 55 but it would probably have a better life in a 90. Research are the key to responsible pet keeping!
For example, you commonly see (not so much on this board but on another... take a guess) when a person puts a few clown loaches in say a 75 and everyone starts to go "OMG get those fish out of there you can't keep them in there for life its overstocked!" when clowns grow so freaking slow they won't outgrow that tank for years.
I personally put myself in the fishes spot. Would I be happy if I was this fish in this tank? Im against catching any wild fish and putting it in a tank for one reason. They go from going where ever they want, to a confined space. Tank bred fish in tanks all there lives really have no idea what "home" feels like. If a human was locked up in a 10x10cell all there life they would think its normal. If you go from traveling the world to a 10x10cell you go insane. off topic but so what! Id say 4 times the length and double there length wide and I think I would be ok. That gives myself a 12x12 tank haha
Its all depends on what kind of fish your keeping and what kind of filtration you have
Really set a size of water change you wan't to do and how often then stock your tank with as many fish as you can untill the water changes and filtration you have can't keep the nitrates and nitrites down once you can't keep those down the tank is over stocked
You could increase the size of water changes you wan't to do and the amount of filtration to increase the stocking limit of your tank but eventually there will come a point where its not prakticle to keep the nitrates and nitrites down this would be the real limit of being over stocked
As long as I can keep the water pristine and the fish can swim I dont consider the tank over stocked. Once it becomes a battle to meet either of those constraints than its over stocked IMO.