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.



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.

I'm in RFC 54 in 1916 and have my setting set to heavy. My forced encounter is off. I want my encounters to be random. I read years ago, I think in Rickenbackers book, where he said at times while flying, he could smell castor oil and knew something had passed recently. That's the way I like to play. Let it just happen and not forced into a fight.

If you examine the mission file deeper, you'll notice half or more flights are either on standby or set to takoff much later in the mission.

Also, with those very high aircraft numbers, you can "stress" test your PC by flying over the front. Select an air start and adjusting the waypoint in the briefing screen to start over the front. Then after a minute, check your FPS.

Yes I have flown missions from start to finish with 800+ aircraft.
But it’s not just this number. There is also much more going on in the game world that determines FPS. Dynamic Campaign!

I need a setting where I can fly any mission, from start to finish, in any weather, and any location ground offensive or no. Finding out on Day 5 of a Campaign, mid-mission, that your computer ain’t cutting it is most annoying. Along with some other FPS tricks I think I’m going to start at Heavy in 1918. If this turns out to be a bad guess I’ll just delete and restart. Ultra Heavy has been great so far in 1917 though a major offensive is upcoming.