[Imp]

If you have air sorties set to XXX in preferences the amount you get varies from battle to battle.
As far as I know visibility, force size, nation & time frame play a part but its then randomised.

Certain dates as in RL conflicts supported by the game give results along the lines of the conflict so if one side had air superiority it would be far more likely to have air availble.

If you specifically want air for a side in a battle then enter the number of sorties you want.
Campaigns always use XXX