Extends Drupal's field cardinality select list 20 values.

mymodule.module

<?php

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