There is no exact amount due to different things. Buy 6 quarts. Put 5, check the dip stick, add as needed until full. Simple as that. One guy may say his car takes 5 one may say his car takes 6. The only way to find out what YOUR car takes is to pour in the minimum, check the level, then add any if needed.
In 2002, Ford changed the owners manual and the dipstick on the 4.6 Mustang motor to require 6 quarts of oil after dealing with excessive warranty claims due to oil starvation when running 5 quarts in the 2V modular motor. Once again, there were no mechanical changes. Ford simply changed the fluid requirement to 6 quarts. If you have a 4.6 built prior to 2002, your dipstick will read full when it is one quart low. Sustained runs at high Rpm's will leave the oil pan dry eventually.