I'm afraid you're doing it a little bit wrong...
Here are the problems:
post_count
field contains the number of posts being displayed and not the offset.wp_count_posts
returns the number of posts globally, not in current query (so it will not be true for queries that have some filters).echo $post_count; . '-' . $post_count;
is not correct PHP code.
So how to do this?
All you need is the info from WP_Query:
<?php
$first_post = absint( ( $wp_query->get('paged') - 1 ) * $wp_query->get('posts_per_page') + 1;
$last_post = $first_post + $wp_query->post_count;
$all_posts = $wp_query->found_posts;
?>
<p class="small text-uppercase">Showing <?php echo $first_post . '-' . $last_post; ?> of <strong><?php echo $all_posts; ?> items</strong></p>