SELECT p_count, count(p_count) AS count_id FROM
    (SELECT bookings.bookings.book_ref, count(bookings.tickets.passenger_name) as p_count FROM
    bookings.bookings INNER JOIN bookings.tickets ON bookings.bookings.book_ref=bookings.tickets.book_ref
    GROUP BY bookings.bookings.book_ref
    ORDER BY p_count desc) z
    GROUP BY p_count
ORDER BY count_id desc
LIMIT 1