How to show the amount of post that have on the site?

Using a query could give you easier customization without having to use SQL directly.

$posts = get_posts( array(
  'post_type'   => 'post',
  'post_status' => 'any', 
  'numberposts' => -1,
  'fields'      => 'ids',
));

$total_count = count( $posts );

print_r ( $total_count );