I've heard different stories on overcrowding...
Lot's of people say 1 inch of fish per gallon of water.... by that standard I'm way over crowded.... I have a 55 gallon tank and 31 fish (or at least I will have that many shortly, I have to replace a few that were struck by ick)
I have talked to many people on the subject and it seems that as long as you hae the right mix of fish (some top feeders, some bottom, and some mid) you can have a lot more than 1 inch of fish per gallon of water... the key there though is that you ned to have enough filtration.
Personally I try to have 1 inch of fish per gallon of filtration. For instance I have a regent powerfilter good for a 60 Gal tank plus a Penguin 330 good for a 70 Gallon tank. that gives me 130 gallons worth of filtration (at least that's how it works in my mind) I am considering upgrading my regent to a Millenium 3000 whic is supposed ot be rated for 100 gallon tank
but I have
4 guppies
5 plattys
5 mollies
1 betta
that are all top feeders
5 sword tails that are mid fish
4 albino rainbow sharks
4 rainbow sharks
3 pictus catfish,
2 golden algea eaters,
2 chinease algea eaters.
Also of importance is to provide plenty of hiding spaces when you have a large number of fish in the tank.
My fish are all healthy and happy (at least the ones who survived the ick)