The volume of the tank on your E-class Coupe C 207 is 8,5L or 24,5L.
When the level of AdBlue® drops to about 3.0L,In case the level of AdBlue® drops to the reserve level of 3.0L, which corresponds to a mileage of 1500 miles, the message “Add AdBlue® see instruction manual" will appear on the dashboard. If you keep on driving without filling up, the message "top-up AdBlue® mileage: ...km" is to be seen on the dial switch. The warning message goes off after 1 minutes after fill up if you drive faster than 10miles/h. If your tank is completely empty, pour a minimum of 3,5 L to switch the warning off.
When your AdBlue® tank is empty and that the engine stops running, it won't restart. if the engine is still running, drive to a service station. After the fill up turn the key in the ignition for about 60 seconds and then start the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.