Write SQL query to solve given problem: How many customers ordered Stephen King's first book?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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