What is the total number of drivers who have cars constructed by Mercedes-Benz?
{'header': ['Year', 'Formula', 'Driver', 'Constructor', 'Location', 'Report'], 'page_title': 'Circuit des Ardennes', 'page_id': '', 'types': ['real', 'text', 'text', 'text', 'text', 'text'], 'id': '1-18893428-1', 'section_title': 'Winners of the Circuit des Ardennes', 'caption': 'Winners of the Circuit des Ardennes', 'rows': [['1902', 'Grand Prix', 'Charles Jarrott', 'Panhard 70', 'Bastogne', 'Report'], ['1903', 'Grand Prix', 'Pierre de Crawhez', 'Panhard 70', 'Bastogne', 'Report'], ['1904', 'Grand Prix', 'George Heath', 'Panhard 70', 'Bastogne', 'Report'], ['1905', 'Grand Prix', 'Victor Hémery', 'Darracq', 'Bastogne', 'Report'], ['1906', 'Grand Prix', 'Arthur Duray', 'Lorraine-Dietrich', 'Bastogne', 'Report'], ['1907', 'Grand Prix', 'Pierre de Caters', 'Mercedes-Benz', 'Bastogne', 'Report']], 'name': 'table_18893428_1'}