The setup is:
18 allied fighters vs. 22 axis fighters (8 Spits, 6 Tiffies, 6 Mossies as pure fighters vs. 8 190's, 14 109's). In frames 3 and 4, the 6 Spits transition to 6 P-51B's; and 8 of the 109's transition to 109G-14's.
The axis is all fighters. No bombers, no ground attack.
The allies also have 12 Mossies for ground attack (generally flown by people who aren't proficient at air-to-air combat).
The scoring is set so that the axis is not expected to be shooting down all (or even most) of the Mossie ground attackers all the time.
Each side does get points for enemies lost in battle (shot down, crashed, ditched, bailed).