What is a typical ratio of aircraft in air vs. total aircraft per airline?
Well here's one way to approach it:

Excluding transatlantic and transcontinental red-eye flights, most airplanes are running between 6 am and about 10 or 11 pm and are down overnight.  Let's just say 18 operational hours per day, or 6570 operational hours per year (we could define operational hours as an aircraft "available for duty").

The average airliner flies between 2 and 3000 hours per year, so lets use 2500.

Being airborne 2500 hours out of 6570 operational hours means the airplanes are airborne about 38% of operating hours.

Allow for aircraft that are in the fleet but are non-operational because they are down for more than overnight; A checks, C checks, D checks, and also dispatch delays.  We could adjust the percentage number down somewhat, say several percent, to allow for that.

We are left with roughly (very roughly, there will be a lot of scatter no doubt) 1/3rd, or slightly better, of a fleet airborne at any one time during the operational day, if you surveyed across a national fleet.