True, and I was expecting 4 separate drops spaced over a few days, not one drop all at once (as you say, this is for some reason impossible regardless of paratrooper or transport plane count).
Just like with other air missions the number of transport planes effectively used for the drop gets scaled down by mission efficiency. This includes air base overcrowding, low supply and bad weather, but also the zone reassignment penalty introduced in 1.5. So if you don't assign the transport planes to any region, you'll get a
massive efficiency hit.
For example in the '39 start, when I let the German paratrooper division drop from the airbase close to Berlin into Warsaw, only 48 out of 132 transport planes can get effectively used, sometimes as low as 18. When assigning the wing to a strategic area first (doesn't need to be the target area), this goes up to 112 effective planes.
Other than that, multiple divisions should be able to drop simultaneously if enough effective planes are available.
As far as I can tell, when a paradrop order is executing and air superiority checks succeed, it collects all deployed air wings with transport capabilities and without assigned missions at the source air base and does the following for each assigned unit:
- check that the unit is (still) parachutable
- check that the unit is at the source air base
- check that the unit is allowed to move into the target province (i.e. military access or war, not water/demilitarized/impassable etc.)
- select a transport wing that still has space and targets the units destination (or didn't have a drop destination yet)
- add the unit to this wings "cargo"
- if the air wing didn't have a drop destination yet, pause its previous region assignment, set its mission to paradrop
Once the air wing has the mission (which is invisible in the interface), it will happily drop off its cargo and reset its mission/region assignment once all cargo is dropped.