Each catapult has a 10% chance of demolishing a level of the wall (and buildings with the appropriate research). However, if more catapults are sent, the game calculates the chances in a special way.
10 catapults will have 100% chance to demolish a level. 12 catapults have 100% chance to demolish one level, and 20% to demolish another. If you send 12 catapults, they will take down at least 1 and sometimes even 2 levels, based on luck.
The research Stone Hail makes it possible for catapults to demolish buildings instead of just the wall. The effect is limited, however, as no building can drop below level 1 and thus cannot be destroyed completely.
Stone Hail catapults are only effective if they survive, while wall-tearing ones take effect even if they get destroyed.