Dynamically add a unique number/identifier at the end of post titles

You will need to leverage the post_updated hook API of WordPress

// You will need to leverage the <code>save post</code> hook API of WordPress
add_action( 'post_updated', 'save_post_callback', 10, 3 );

//add_action('save_post', 'save_post_callback');
function save_post_callback($post_ID, $post_after, $post_before){
    global $post;
    if ($post->post_type != 'page'){
        return;
    }

    // If you get here then it's your post type so do your thing....
    // unhook this function so it doesn't loop infinitely
    remove_action('post_updated', 'save_post_callback');

    $title = $post_after->post_title;
    // Check if "NTS00" is already there,
    // Deliberately checking if position found is > 0
    if( strpos($title, "NTS00") == FALSE ){
        // Set the title
        $args = array(
            'ID'           => $post_ID,
            'post_title'   => $title.'_NTS000'.$post_ID
        );
        // update the post, which calls save_post again
        wp_update_post( $args );
        // Bonus! 
        update_post_meta( $post_ID, 'my_key', 'NTS000'.$post_ID );
        // re-hook this function
        add_action('post_updated', 'save_post_callback');

    }
}

This way you will ensure that the NTS00 gets appended only once.
Bonus feature will ensure that you also store that value for a “custom key” as post meta.

I hope I was able to help.