Drupal 8 pass data down ER fields

example.module.php

<?php

/**
 * @file
 * An example of how to pass the item number from a multi-value
 * entityreference field to the entities concerned.
 */
 
/**
 * Implements hook_preprocess_field().
 * Change `field_name` to your field machine name.
 */
function example_preprocess_field__field_name(&$variables) {
  foreach ($variables['items'] as $key => $item) {
    $variables['items'][$key]['content']['#field_item_number'] = $key + 1;
  }
}

/**
 * Implements hook_preprocess_node().
 */
function example_preprocess_node(&$variables) {
  if (isset($variables['elements']['#field_item_number'])) {
    $variables['field_item_number'] = $variables['elements']['#field_item_number'];
  }
}

node.html.twig

{#
// .. usual template stuff
#}
<article>
{% if field_item_number %}
  This is item number {{ field_item_number }} in the field which references it.
{% else %}
  {{ content }}
{% endif %}
</article>