I'm thinking it's like, # of Pokemon in match x Attack Power x Super effectiveness/non effectiveness. Am I right, or how else is it?
The base damage you do is the Pokemon's Attack power. So a level 1 Haxorus has 70 attack and will do 70 points damage normally with a 3-icon match.
There are several multipliers though:
There are different ways of calculating it. This source tells you most, but not all abilities. But still pretty useful: