How many customers ordered Stephen King's first book?
"Stephen King" is the author_name; first book refers to Min(publication_date)
SELECT COUNT(T1.publication_date) FROM book AS T1 INNER JOIN book_author AS T2 ON T1.book_id = T2.book_id INNER JOIN author AS T3 ON T3.author_id = T2.author_id INNER JOIN order_line AS T4 ON T4.book_id = T1.book_id WHERE T3.author_name = 'Stephen King' ORDER BY T1.publication_date ASC LIMIT 1