How many animation film titles are rated for adults only?
animation film refers to category.name = 'animation'; for adults only means rating = 'NC-17'
SELECT COUNT(T1.title) 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 = 'animation' AND T1.rating = 'NC-17'