I had a look, and here is what I think would control the amount of air at any time
there is a section under difficulty
AI spawn sped scaling RED ,Multiplies non-supply AI attack and patrol timers
normal is 1 if set to 1.5 it is 50% slower speed on spawning
Im not sure if this includes supply helicopters a there is a separate supply speed scaling under logistics/warehouse
hope this is right and helps