How to display the number of posts in category on category archive page?

I got a code from

<?php
$cat = get_query_var('cat');
$categories=get_categories('include='.$cat);
if ($categories) {
  foreach($categories as $category) {
    echo 'Posts in this category = ' . $category->count;
  }
}
?>

My question is where to paste these code? Sorry I am a noob…