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I'm afraid you're doing it a little bit wrong...

Here are the problems:

1. post_count field contains the number of posts being displayed and not the offset.
2. 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).
3. 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('page') - 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>