The volume of the tank on your C-class Saloon C205 is 8,5L or 24,5L.
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. Later, the message "top-up AdBlue® mileage: ... miles" is to be seen on the dial switch. You can still drive the indicated mileage. Eventually, the message "Top-up AdBlue®, restart impossible" is displayed to announce breakdown. Once the tank is empty, pour a minimum of 3,5L to switch off the warning.
As soon as your tank is empty and that you turn the engine off, you cannot restart the car. If the engine is still running, you are able to drive to a service station. After fill up, turn the ignition on for 60 minutes and start the engine.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.