How many actors acted in movies in the Music category?
Music' is a name of a category
SELECT COUNT(T1.actor_id) FROM actor AS T1 INNER JOIN film_actor AS T2 ON T1.actor_id = T2.actor_id INNER JOIN film AS T3 ON T2.film_id = T3.film_id INNER JOIN film_category AS T4 ON T3.film_id = T4.film_id INNER JOIN category AS T5 ON T4.category_id = T5.category_id WHERE T5.name = 'Music'