The volume of the tank on your CLS-class Shooting Break C218 is 8,5L or 24,5L.
If the level of AdBlue® reaches the reserve level of 3,0 L, which corresponds to a 1500 miles autonomy, the message "Add AdBlue® see instructions manual" appears on the dashboard. When the minimum level of AdBlue® is reached, the message "Add AdBlue® autonomy: ... miles" appears to show your mileage before breakdown. Lastly, the message "Add AdBlue, starting impossible" appears to point out the breakdown. When the tank is empty, pour at least 3,5L to make the signal disappear.
If the signal appears, you have to fill up with AdBlue® as soon as possible. When the tank is empty, you can keep driving as long as the engine doesn't shut down. After filling up, turn on the ignition for at least 60 seconds and restart the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.