There are many points to take under consideration when deciding on stocking levels. I prefer to keep it on the low side so if there is a power cut, or something goes wrong, things don't go so bad so fast.
The better your filtration and aeration, the more plants and the more water changes you do regularly all help you keep higher stock levels but if only one thing goes wrong (heaterstat dies, filter stops, you hurt your back and are unable to do a water change etc) then the whole thing can get very toxic very quickly.