Answer:
According to the 2005 Ford Mustang Owner Guide :
The cooling system capacity is 13.6 quarts ( 12.9 liters )
So , a 50 / 50 mix of antifreeze and preferably distilled water would require :
( 6.8 quarts / 6.45 liters of antifreeze )
* Ford shows not to use less than 40 % antifreeze and not to exceed 60 % antifreeze in the cooling system mixture *