I would get 6 quarts if you are going to do it that often. We have GM and I have never got more than 6-7 quarts out of one from the pan. I like to chance the old out and add the new without them mixing so do the Bucket Flush by tapping in to the radiator cooler return line but your method every few months sounds like a good plan.
How much will drain out will depend on how long you let it set and how much higher the front tires are than the rears due to the torque converter holds a lot of ATF and takes time to drain down as much as possible.