views-view-unformatted--home-news.html.twig

Within {% for row in rows %} we can use {{ loop.index }} like: <article id="newsitem{{ loop.index }}">.

Full output might look like:

{% for row in rows %}
  <article id="newsitem{{ loop.index }}" class="newsitem col-md-4 col-lg-4 col-sm-6 col-xs-12">
    {{- row.content -}}
  </article>
{% endfor %}