How to limit post of custom post status?

You can use one of the status transition filter hooks for this:

• {$new_status}_{$post_type}
• transition_post_status

In the function assigned to the hook, you change the status to publish the previously featured post ( using e.g. $wpdb ).
Both actions are performed after saving the post, so you have to change the status in posts other than edited.

add_action( 'featured_commercials', 'se339582_single_featured', 10, 2 );
function se339582_single_featured( $post_id, $post )
{
    global $wpdb;

    $sql = $wpdb->prepare( "UPDATE {$wpdb->posts} SET post_status="publish" "
            ." WHERE post_status="featured" AND id <> %d", $post_id );
    $wpdb->query( $sql );
}

Update:

SQL for one featured post per category:

$taxonomy_slug = 'commercials';
$sql = $wpdb->prepare( "UPDATE {$wpdb->posts} p " .
    " INNER JOIN {$wpdb->term_relationships} tr ON tr.object_id = p.id " .
    " INNER JOIN {$wpdb->term_taxonomy} tt ON tt.term_taxonomy_id = tr.term_taxonomy_id " .
    " SET p.post_status="publish" " .
    " WHERE p.id <> %d AND p.post_status="featured" AND tt.taxonomy=%s AND tt.term_id = %d", 

    $post_id, $taxonomy_slug, $term_id
);