Two ways to know volume
If you like to do it the hard way, Just re-measure the volume of the drained fluid by pouring it into an empty quart and reapeat the process until all the old fluid is measured. Ofcourse this is not the best way as your fluid may be low before the drain. Or, look at your user's manual and see how many quarts need for a complete drain. usally about 6 quarts.
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer