hugo-theme-flat/layouts/partials/side-tags.html

<div class="side-tags">
    <h2>Tags</h2>
    <hr />

    <ul>
        {{ range .Site.Taxonomies.tags.Alphabetical }}
            <li>
                <a href="{{ "tags/" | relURL }}{{ .Name | urlize }}">{{ .Name }} ({{ .Count }})</a>
            </li>
        {{ end }}
    </ul>
</div>