How many times did Saxby Chambliss have a lead margin of 21?
{'header': ['Poll Source', 'Dates administered', 'Democrat: Jim Martin', 'Republican: Saxby Chambliss', 'Lead Margin'], 'page_title': '', 'page_id': '', 'types': ['text', 'text', 'text', 'text', 'real'], 'id': '1-16751596-16', 'section_title': '', 'caption': '', 'rows': [['Public Policy Polling', 'November 2, 2008', '46%', '48%', '2'], ['Survey USA', 'November 2, 2008', '44%', '48%', '4'], ['Rasmussen Reports', 'October 30, 2008', '43%', '48%', '5'], ['Rasmussen Reports', 'October 22, 2008', '45%', '47%', '2'], ['Survey USA', 'October 12, 2008', '43%', '46%', '3'], ['Rasmussen Reports', 'October 7, 2008', '44%', '50%', '6'], ['Research 2000/Daily Kos', 'October 1, 2008', '44%', '45%', '1'], ['Survey USA', 'September 29, 2008', '44%', '46%', '2'], ['Rasmussen Reports', 'September 16, 2008', '43%', '50%', '7'], ['Survey USA', 'September 14-September 16, 2008', '36%', '53%', '17'], ['Strategic Vision', 'September 7-September 9, 2008', '36%', '54%', '16'], ['Rasmussen Reports', 'August 14, 2008', '44%', '50%', '6'], ['Rasmussen Reports', 'July 17, 2008', '40%', '51%', '11'], ['Strategic Vision', 'June 27-June 29, 2008', '28%', '57%', '29'], ['Rasmussen Reports', 'June 26, 2008', '39%', '52%', '13'], ['Rasmussen Reports', 'June 4, 2008', '36%', '52%', '16'], ['Rasmussen Reports', 'May 6, 2008', '33%', '54%', '21']], 'name': 'table_16751596_16'}