How many films in the horror category were included in PG-13-rated?
"Horror" is the name of category; PG-13 rated refers to rating = 'PG-13'
SELECT COUNT(T1.film_id) FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T2.category_id = T3.category_id WHERE T3.name = 'Horror' AND T1.rating = 'PG-13'