How many quarts of oil does my car take?

Service information shows 8 liters (8.45 Quarts) with a filter change, 7.5 liters (7.92) if you don't change the filter So you will want to buy 8-9 quarts plus the filter.
As for the process used, Mercedes wants you to use an oil extractor on their engines and pull the oil out through the dipstick tube with the engine hot. You run the engine until it's warmed up, shut it off and get the tools all set up, you need at least 5 minutes to let the oil fully drain back to the pan, then you use the oil extractor and while it is working you change the filter. They don't recommend pulling the drain plug because they are easy to damage, and if it has been changed using an extractor until now it will very likely damage the oil pan on removal.
For the filter, wipe off the plastic housing, then remove the filter housing cover. Have a rag or tray handy so that when you pull the cover up the filter doesn't drip on everything. Remove the old filter, wipe the surface of the cap and housing off, replace the O-rings and apply a small amount of oil to the seals, then install the filter and tighten the cap back in place. The cap is tightened to 25 Nm or about 18 ft lb. Don't try to tighten it very hard as they are easy to crack. Now refill the oil. Start the engine and run it for about 30 seconds, and check the oil level.