I recommend one in the forward berth area in case you are sleeping when a fire starts, and one near the main cabin door so you can easily grab it on the way out the door. That also puts the second one close to the galley, but far enough away that if you have to jump back from a galley fire, you can still grab it quickly.
I mounted our forward berth extinguisher to a cabinet so the screws would penetrate inside the cabinet, and the one near the door down in the mid berth so the screws would penetrate into a storage locker. That way I could be sure there were no wires, hoses, or mechanical/electronic devices in the way. As it turns out, the lamination is thick enough that the screws didn't penetrate.