Once the game size / planes have been determined, one (or more) squadron may be selected to fill the game.
As it's not necessarily a balanced 1 squadron vs 1 squadron, it is not necessary for the squadron to be of any size.
You can even have a 2-people squadron: if the game contains only 2 planes of a side, that squadron will fly alone against the enemy, otherwise another squadron or non-squadron people might join them.

Also, squadrons can be divided into smaller flights as flying the whole squadron all the time would mean very large games for some squadrons.