Here's the reg...There is no mention of an altitude limit....that limit may be effected by the size of your check book...
High Altitude Aircraft Endorsement ยง61.31
No person may act as pilot-in-command of a pressurized aircraft capable of operating at high altitudes unless that person has received and logged ground and flight training from an authorized instructor and received a one-time endorsement certifying the pilot has satisfactorily completed the ground training and is proficient to operate a pressurized aircraft.
A pressurized aircraft capable of operating at high altitude is defined as an airplane with a service ceiling or maximum operating altitude; whichever is lower, above 25,000 feet MSL.
Ground training must include at least the following subjects:
High-altitude aerodynamics and meteorology
Respiration
Effects, symptoms, and causes of hypoxia and any other high-altitude sickness
Duration of consciousness without supplemental oxygen
Effects of prolonged usage of supplemental oxygen
Causes and effects of gas expansion and gas bubble formation
Preventive measures for eliminating gas expansion, gas bubble formation, and high-altitude sickness
Physical phenomena and incidents of decompression
Any other physiological aspects of high-altitude flight
Flight training must include at least the following subjects:
Normal cruise flight operations while operating above 25,000 feet MSL
Proper emergency procedures for simulated rapid decompression without actually depressurizing the aircraft
Emergency decent procedures
The flight training may be done in a flight simulator or flight training device that is representative of a pressurized airplane. The pilot-in-command must be proficient in the operation and systems of the airplane.
The training and endorsement is not required if that person can document satisfactory accomplishment of any of the following in a pressurized aircraft, or in a flight simulator or flight training device representative of a pressurized aircraft:
Serving as pilot in command before April 15, 1991
Completing a pilot proficiency check for a pilot certificate or rating before April 15, 1991
Completing an official pilot-in-command check conducted by the military services of the United States
Completing a pilot-in-command proficiency check under part 121, 125 or 135 conducted by the Administrator or by an approved pilot check airman.
An example of the required endorsement is shown below.
To act as PIC in a pressurized aircraft capable of high altitude operations: section 61.31(g).
I certify that (First name, MI, Last name), (pilot certificate), (certificate number), has received the required training of section 61.31(g) in a (make and model of pressurized aircraft). I have determined that he/she is proficient in the operation and systems of a pressurized aircraft.