The general rule of thumb on the number of fish a tank can support is:
1" fish = 1 gallon of water.
I recently had 13" inches of fish in a five gallon w/ a powerhead filter (they were all much smaller when I put them in - the little buggers outgrew the tank fast). Even w/ excellent filtration and regular water changes, it lead to water quality issues and illness.
The fish in question were moved into a much big tank and the problem was solved, but not before my betta got popeye and paid with part of her eyesight.
We try not to overstock tanks, to avoid such issues.