The volume of the tank on your C-class Saloon W204 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. The warning message goes off after 1 minutes after fill up if you drive faster than 10 miles/hour. If the level reaches its minimum, the message "top-up AdBlue® mileage: ...km" is to be seen on the dial switch indicating to you your remaining mileage. In case of a breakdown, the message "Top-up with AdBlue®, restart impossible shows up to ask you you to top up. If your tank is completely empty, pour a minimum of 3,5 L to switch the warning off.
As soon as you see the AdBlue® warning light, you should refill your AdBlue® tank so as to be able to restart the engine. If you do not fill up with AdBlue®, you will not be able to restart your car. 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.