Technically if you want to use species counts as well, then I have a massively overstocked 100 gallon tank right now, which is serving as a grow out tank for fish all under a few months old:
215 Pytchochromis Oligocanthus (It's a substrate spawner, huge numbers are a given when spawning)
65 Pseudotropheus Flavus (Several breeding adult females in my 180)
10 Labidochromis Caeruleus (No breeding going on anymore, just growing up younger ones right now)
25 Protomelas Steveni Albino "Taiwan Reef" (Mom is a very prolific breeding in that I have another 30 or so in my 15 that are 2 weeks old just waiting to get size to move up to that 100)
3 Astatotilapia Aeneocolor (Not big enough to go in the 75 just yet)
20 Metriclimea Lombardoi (Almost ready to move to the main tank)
10 Astatotilapia Lafasciata (netted by accident trying to move the Lombardoi up last week, smallest in the tank but doing well)
Technically by sheer numbers this is overstocked big time, but it's a fry grow out tank. Not one fish in there is bigger then 1.5 inches right now, they can't go in the 180, I have a full grown male Nimbochromis Venustus in there who would make meals out of them at the current sizes, and the 15 really can't cope with that much anyways. 75 gallon is populated with 6 month old Labidochromis "Red Top" Hongi, Astatotilapia Aeneocolors, Labidochromis Caeruleus from a different bloodline, a small pair of Paratilapia Polleni and some assorted Peacocks. There isn't much choice but to use the 100 as a staging tank. I think overstocking in this case is a necessity and it's basically temporary. Many of these species will get moved to different tanks or sold/traded in the next few months, then the next cycle of spawned fish move in from the 15, holding females to the 15 and the cycle starts all over again.