This is only a US and possibly other US areas, in Europe we use "persons" not souls.
This is used when asking a pilot for details of the amount of people on board his/her aircraft. SOB and POB (Souls on board) (People on board).
This was derived from nautical terminology where they were slightly ambivalent to define who was actually alive.
Souls....
steward
They are called flight attendants. They serve the passengers with food, alcohol and other things. They help passengers escape in the time of disasters of accidents.
Male OR female are called the same - Cabin Crew
Airport
Passengers sit in seats in the' fuselage' of the aircraft
500
The accepted job title is "flight attendant". It applies to both men and women.
A small airplane can fit upto 4 passengers.
Safer airplanes
420
The original Wright Flyer held no passengers, only the pilot.