Max manifold pressure

The 912IS is a "Naturally aspirated" engine ie NOT turbo charged. Therefore it will be designed to run at all the natural air pressures on planet Earth  with a little margin for safety. Being EC controlled the ECU will as as needed change the fuel and timing accordingly. So anything up to 30 inches or say 32 would be compensated for.

IF you want to turbocharge a IS then it depends on whether the engineers bothered to program the ECU to go much higher, you may be lucky and the ECU may not be the limit but knocking might be.