Question: Each time I modify a POST, a copy of old POST is created and saved. Can I control, how many old posts to retain? Can I clean all old posts?
Answer: Yes! You can stop collecting old versions of the posts by adding this line to your wp-config.php file:
define('WP_POST_REVISIONS', false);
And Yes! You can delete all “revisions” by running this SQL statement against you database:
DELETE wp_posts.post_type FROM wp_posts WHERE wp_posts.post_type = "revision"