A preprocess function that adds count wrappers in Drupal regions.

D8 Region Preprocess Classes

use Drupal\Component\Utility\Html;
use Drupal\Core\Render\Markup;
use Drupal\Core\Render\Element;

/**
 * Implements hook_preprocess_region().
 */
function MYTHEME_preprocess_region(&$variables) {
  $elements = $variables['elements'];
  $region_children = Element::children($elements);
  $variables['region'] = $elements['#region'];
  // Utility regional classes for $content.
  $variables['content'] = array();
  foreach ($region_children as $k => $v) {
    $region_classes = Html::cleanCssIdentifier('region--' . ($k + 1) . '-of-' . count($region_children));
    $variables['content'][] = '<div class="' . $region_classes . '">' . $variables['elements'][$v]['#markup'] . '</div>';
  }
  $variables['content'] = Markup::create(implode($variables['content']));
}