About 4 quarts. When you drain from the drain plug, you cannot drain the torque converter so you do not get the entire 10 quarts. Some do a double drain, with a short drive between, to get a larger percentage of the fluid changed. If you want to change all the fluid you will need to use the hose in a bucket method and flush the trans. This has been covered many times before and you can probably find out about it with a search.
