Maximun is 45,000 ft under ATC, Absolute Maximun is 60,000 without special permission from the FAA
because it has been pressurised in a similar way (outcome) to how a commercial airplane is pressurised for passengers to fly at altitude.
The Blackbird
It depends on what kind of jet or commercial airplane it is.
It depends what aircraft and what speed you are going
an added tank
There is no certain cruising altitude for a commercial airline. it depends on where they are flying and how long they fly. most airlines fly above 10,000 ft. the max altitude is around 41,000 ft.
stratosphere
The Concorde's highest cruising altitude was approximately 60,000 feet (about 18,300 meters). This altitude allowed it to fly above commercial air traffic and weather, contributing to its ability to achieve supersonic speeds of over Mach 2. The high cruising altitude also provided passengers with stunning views of the curvature of the Earth.
22 hours
The u2 spy plane it can fly so high you and see the curveture of the earth
Above 50,000 feet (15 kilometers)
10 kilometers (about 32,808 feet).