My 420 SEL required around 7.75 to 8 quarts of gearbox fluid.
The instructions instructs to first pour 4 litres to start the vehicle, then gradually add the remainder while changing gears.
I would refrain from adding the remainder, since it may lead to overfilling.
To do the final adjustments, operate the gearbox at full temperature and drive the vehicle for 20 minutes. The adjustment range should be around 200 mL between the minimum and maximum levels. It is advisable to begin with a quantity slightly below the minimum level.
It is simpler to add than to delete.