Drupal: Change Multiple Field Values bigger than 10 and smaller unlimited

mymodule.module

/**
 * Implements hook_form_alter().
 */
function MODULENAME_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'field_ui_field_edit_form') {
    $max_values = 20;
    $form['field']['cardinality']['#options'] = array('-1' => t('Unlimited')) + drupal_map_assoc(range(1, $max_values));
  }
}