The volume of the tank on your S-class W222 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.If you don not fill up, the message "top-up AdBlue® mileage: ...km" is to be seen on the dial switch indicating to you your remaining mileage. 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.
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, with an empty tank, you are able to drive to a petrol 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.