In terms of mbuna-keeping, "overstocking" and "too many" do not necessarily mean the same thing (assuming that you are stocking mbuna's, which you may or may not be). Overstocking mbuna's is a way to minimize aggression (though not the only way so it is not necessarily the way to go).
8-12 is probably adequately stocked. Less than 8 would be considered understocked which can lead to aggression issues depending on the species and specimens involved.
Overstocking for a 55gal would be approaching or exceeding 18-20 (give or take).
If you're dealing with non-mbuna's and especially non-african cichlids then none of the above may apply.