Two layers of 9mm used to be quite common in UK if you are aiming for a robust, heavy, quality feeling wall. One layer of 12mm feels cheap, not that hard to put

Two layers of 9mm used to be quite common in UK if you are aiming for a robust, heavy, quality feeling wall. One layer of 12mm feels cheap, not that hard to put a hole in it while moving furniture etc. One layer of 15mm board is common now though.