The rulebook says: 1 catapult per region in the Pelennor not adjacent to Minas Tirith. That's 3 regions. Then it says you can attack with all catapults on the board with an army die, with a maximum of 5 catapults per attack. So are you allowed to build more than one catapult per region?
As I said, the rulebook says that catapults can be placed in Pelennor-regions not adjacent to Minas Tirith, "not more than one catapult per region". Maybe a wrong translation? I hope the complete rules will be published soon.