How to numerate drupal views rows in twig?
The fields template is probably not the best place. Normally you would do this in the views style plugin. Grid has such options built in, for Unformatted you can add

loop.first ? 'views-row-first',


to  views-view-unformatted.twig.html:

{% if title %}
  &lt;h3&gt;{{ title }}&lt;/h3&gt;
{% endif %}
{% for row in rows %}
  {%
    set row_classes = [
      default_row_class ? 'views-row',
      loop.first ? 'views-row-first',
    ]
  %}
  &lt;div{{ row.attributes.addClass(row_classes) }}&gt;
    {{ row.content }}
  &lt;/div&gt;
{% endfor %}