How many keywords does the episode that was aired on 2008/10/19 have?
aired on 2008/10/19 refers to air_date = '2008-10-19'
SELECT COUNT(T2.keyword) FROM Episode AS T1 INNER JOIN Keyword AS T2 ON T1.episode_id = T2.episode_id WHERE T1.air_date = '2008-10-19';