It's always a question of how many fish can be managed in a given size aquarium. Now for a moment, lets set aside that some species simply REQUIRE many of their own kind in order to be content...and lets just focus on bio-load relative to tank size.
I'll just cut to the bottom line. It all really comes down to water quality. With just a few fish, especially in a well planted, balanced tank, you can get away with little routine maintenance since water quality degrades very, very slowly. You can have what most would consider to be an overstocked tank, if, and only IF, you do what's necessary to maintain high water quality.
IF you have plants, especially fast growing floating plants, feed modestly, AND do routine partial water changes of sufficient volume/frequency to ensure high water quality, then you can have a 'well stocked' aquarium. BUT, you can't slack off as that would be potentially harmful to the fish you are the caretaker for. Like any other animals in our charge, we have to be responsible to give our fish the best life possible!
Good Fishkeeping.