For season 9, episode 17 of the show Law and Order, how many roles have been included in the credit?
Law and Order refers to series = 'Law and Order'; included in the credit refers to credited = 'true'
SELECT COUNT(T2.role) FROM Episode AS T1 INNER JOIN Credit AS T2 ON T1.episode_id = T2.episode_id WHERE T1.series = 'Law and Order' AND T1.season = 9 AND T1.episode = 17 AND T2.credited = 'true'