Spoiler alert. Do not click below unless you want to know how Air Activity effects aircraft numbers in the game.
Normally I do not wish to know, too deeply, how things work in the game but everyone knows this fact just not the exact numbers. I needed a baseline to determine how high I could go in 1918 with AA levels and still have acceptable FPS.
Numbers courtesy of JJJ65's Mission Editor.
Set up:
21 March 1918 - the first day of the great German Offensive aka "Kaiserschlacht." Figured big numbers.
Flying with Jasta 11 starting a flight, quitting, getting the aircraft number, then deleting the career and doing another changing the Air Activity Level.
Only difference between the careers was the mission assignment, in-game morning start time, and slightly (very slightly) different weather.
Ultra Heavy: 798 aircraft. Of interest about 2 weeks before the offensive was to start (in another career) I had 879 aircraft. The subsequent lag fest started me looking.
Very Heavy: 543 aircraft.
Heavy: 344 aircraft.
Medium: 195 aircraft.
Light: 149 aircraft.
Very Light: 75 aircraft.
Because of the couple dozen variables I didn't concern myself with FPS other then a quick glance. But, right at mission start, less aircraft does not always mean better FPS *at that moment.*
Of course a different squadron in a different time and different place will have different numbers. Even if you were silly enough to waste 40 minutes of your life duplicating what I did here I suspect your numbers would be still be different.
And in-game FPS is not just about aircraft numbers (I learned from previous tests) but its a good start. With all that said this may be considered a general guide reference the proportionality of the different settings.