From what I understand I think you can try something like:
SELECT meta_value, COUNT(post_id) as count FROM $wpdb->postmeta WHERE meta_key = 'authors' GROUP BY meta_value HAVING count >= 20 ORDER BY meta_value;
Let me know if this works for you!
From what I understand I think you can try something like:
SELECT meta_value, COUNT(post_id) as count FROM $wpdb->postmeta WHERE meta_key = 'authors' GROUP BY meta_value HAVING count >= 20 ORDER BY meta_value;
Let me know if this works for you!