The specs I looked up say initial fill is 6.1 quarts or just over 1.5 gallons. If thats true you'll get much less just from draining the radiator. Probably a gallon of 50/50 should cover you unless you're doing flush?
For future reference, the Amsoil Online Product Application Guide is an excellent resource for finding capacities and fluids specs. As noted total system initial fill, 6.1 qts.