That depends on the Paint, Reduction, Airbrush, what you are painting on, and what you are painting.
15-20 I found is good for just about everything when doing illustration on hardboard or illustration board.
I do slam it up to 40-50 if spraying backgrounds with unreduced paint, or blowing out tip dry.
Answer the questions above, and we can give you a better idea of what PSI to use.
Static pressure is what you set the regulator on wben you are not spraying. Whenypu start spraying the pressure will drop. Air is flowing and coming out the tip. Static is no air flowing. Like a water faucet when the water is off the water pressure is static, no water flowing. When you turn on the water the pressure will drop to a flowing psi.