Some new official info from ubi forums

[Blackdog_kt]

To let the user increase this in a scripted campaign, they would have to make each mission at least 3 times to account for a bare minimum of low/medium/high aircraft density settings.

There's nothing stopping me from creating my own scenarios with 100 bombers in the air, or flying online in a server with a few dozen people if my PC can handle it.

Nevertheless, when the dynamic campaign is ready it would be a cool feature to be able to select the desired aircraft density.