Name the total number of episodes written by david cantor
{'header': ['Episode', 'Original title', 'Directed by', 'Written by', 'Original airdate', 'Duration', 'Viewership'], 'page_title': '', 'page_id': '', 'types': ['real', 'text', 'text', 'text', 'text', 'text', 'text'], 'id': '1-17641206-6', 'section_title': '', 'caption': '', 'rows': [['16', '" But is it Art? "', 'Dewi Humphreys', 'Jim Sullivan', '2November2007', '30 minutes', '5.34 million'], ['17', '" The Lonely Herdsman "', 'Dewi Humphreys', 'Jim Sullivan', '9November2007', '30 minutes', '4.62 million'], ['18', '" If You Go Down to the Woods "', 'Dewi Humphreys', 'Jim Sullivan', '23November2007', '30 minutes', '5.20 million'], ['19', '" Sweet Sorrow "', 'Dewi Humphreys', 'John Sullivan and Keith Lindsay', '30November2007', '30 minutes', '6.05 million'], ['20', '" Fifteen Minutes "', 'Dewi Humphreys', 'David Cantor', '7December2007', '30 minutes', '5.08 million'], ['21', '" The Final Curtain "', 'Dewi Humphreys', 'John Sullivan', '14December2007', '30 minutes', '5.46 million']], 'name': 'table_17641206_6'}