How many gallons of coolant do you need to flush radiator of a 1997 Chevy lumina?

First use clean water to flush the system then a mixture of 50% clean water and 50% antifreeze to refill the system if you were able to drain the entire system. The cooling system in an average vehicle will hold aprox 10 to 12 quarts. The block drains can be difficult to remove to drain the block dry so if that is your case, after you have flushed the system with clean water add 4 qts of straight antifreeze to the radiator. This will mix with the clean water in the engine block giving you a 50/50 mix. If you need to add more coolant after that use the 50/50 mix.