The amount of fluid required in a flush varies between 10 to 20 quarts depending on the size of the transmission in the vehicle. Larger engines use bigger transmissions and as a result require more fluid to properly fill the gearbox.
A transmission flush is a procedure required to change out the old fluid inside of a gearbox. When the fluid is too old, the transmission will have trouble shifting through gears and may cause the vehicle to stall. On average be expected to use between 10 to 14 quarts of transmission fluid on smaller vehicles and 14 to 20 quarts on larger trucks in a normal flush procedure.