Валидация поля. Если количество элементов менее 2. Получение количества IDs

basf_site.module

/**
 * Validate Carousel gallery paragraph.
 *
 * @param array $element
 *   Reference form array.
 *
 * @param \Drupal\Core\Form\FormStateInterface $formState
 *   Form state object.
 */
function basf_site_validate_carousel_gallery(
  array &$element,
  FormStateInterface $formState
): void {
  $subItems = $element["subform"]["field_gallery_slides"]["widget"]["selection"];

//  Gets count of slides.
  $str = implode(",", array_keys($subItems));
  $subItems = preg_replace('/[^0-9.]+/', '', $str);

  if (count(str_split($subItems)) < 2) {
    $error = t('At least 2 media sliders is required.');
    $formState->setError($element, $error);
  }
}