0 to hundreds -depending on the model
Depends on the airplane. Some really small ones carry only the pilot but some of the really large wide bodies can carry as many as 300.
525 people in standard three-class configuration, or up to 853 people in all economy class configuration.
2 at a time
Two people at most. The first airplane carried its engine and 1 man.
About 23 US gallons
Bombardier makes a 50-passenger jet. Some Boeing 747s carry more than 500. The Airbus A380 will carry more than that.
There are many people that improved the airplane. The airplane is still being improved today.
Yes, you can.
A gun
guns
Carefully.