how many quarts of fluid do I replace when I change transmission filter on 2006 chevy impala

Measure what you take out and put back the same amount and recheck after driving a bit. These systems hold about three gallons but you will only see five or six quarts getting replaced by dropping the pan to change the filter. Don't buy the fluid until you see how much you really need. Then buy a gallon to start and a few quarts to finish up. (Cheaper by the gallon.) And naturally, don't run the motor or drive the car until it's refilled with the correct tranny fluid. Have fun.