getSelectedMediaItemCount example

  public function updateLibrary(array &$form, FormStateInterface $form_state) {
    if ($form_state::hasAnyErrors()) {
      return $form;
    }

    $media_ids = array_map(function DMediaInterface $media) {
      return $media->id();
    }, $this->getAddedMediaItems($form_state));

    $selected_count = $this->getSelectedMediaItemCount($media_ids, $form_state);

    $response = new AjaxResponse();
    $response->addCommand(new UpdateSelectionCommand($media_ids));
    $media_id_to_focus = array_pop($media_ids);
    $response->addCommand(new ReplaceCommand('#media-library-add-form-wrapper', $this->buildMediaLibraryUi($form_state)));
    $response->addCommand(new InvokeCommand("#media-library-content [value=$media_id_to_focus]", 'focus'));
    $available_slots = $this->getMediaLibraryState($form_state)->getAvailableSlots();
    if ($available_slots > 0 && $selected_count > $available_slots) {
      $warning = $this->formatPlural($selected_count - $available_slots, 'There are currently @total items selected. The maximum number of items for the field is @max. Please remove @count item from the selection.', 'There are currently @total items selected. The maximum number of items for the field is @max. Please remove @count items from the selection.', [
        '@total' => $selected_count,
        '@max' => $available_slots,
      ]);