It takes 6.3 quarts. Check your owner's manual. Start with 6 quarts and carefully top up. Don't overfill!
I have a low mileage 300E and it gets over 4K on a quart of Mobil 1 15W-50.
Don't trust mechanics. Check everything they do. I once had a mechanic at a MB dealership try to put over 7 quarts of oil in my M103 engine. He got it confused with the M104. That isn't the only thing I've caught them at.