What is the highest theoretical and current flight a helicopter can achieve?
This (and more) is quite easily answered by Wikipedia
Highest altitude without payload 12,442 m (40,820 ft) Aerospatiale Lama
Highest level flight altitude 11,010 m (36,120 ft) Sikorsky CH-54 Tarhe
Altitude with 40-tonne payload 2,255 m (7,398 ft) Mil V-12
Visit the linked Wikipedia article for more records and sources.