Here is the link to the tech article: Link
In the section where it actually describes the changing process it says:
"On 4-cylinder cars, you will need about 7 quarts (6.5 liters). For the six cylinder cars, you will need 10.6 quarts (10 liters). M3 motors require 11.1 quarts (10.5 liters)."
Buy the coolant at BMW and mix 50/50 with distilled water.