When there's an active airplane integration, GSX will load the exact number of passengers as specified by the airplane, when it's not, it will estimate a number depending on the airplane payload (unless you have a Simbrief flight plan), which means:
- If the airplane integration is active, and you have only 1 passenger loaded, it means the airplane has told GSX it has 1 passenger.
- If the airplane integration is not active, and you have only 1 passenger loaded, it means the airplane is empty, so GSX estimated only 1 passenger.