In my opinion as long as ammonia and nitrite are still 0 ppm and nitrate is under 30 ppm (or whatever your particular fish species requires. Discus would have problems at much less than 30 ppm while bluegills wouldn't bat an eyelash at 100 ppm), so as long as the water is healthy for the the fish that are in it, the fish tank is not overstocked.
Oh, also, the fish has to be able to turn around.
But yeah, other than that it's all up to you what kind of tank you want. Fish don't live all perfectly spaced out in the wild; social species travel together and clump all in one spot. Check out this video of fish density in a channel: http://www.youtube.com/watch?v=z9upkl4rBfU