js/js.js
<script>
 jQuery(document).ready(function($){
                  var count = $("#jform_params_foreignmanuf :selected").length;
                  return count;
              });
</script>

manulist.php (dropdownlist)

<?php

defined('JPATH_PLATFORM') or die;
JFormHelper::loadFieldClass('list');
class JFormFieldManulist extends JFormFieldList
{

    protected $type = 'Manulist';


    protected function getOptions()
    {

     $db= JFactory::getDbo();
     $query = $db->getQuery(true);
        $query->select('*');
        $query->from('#__virtuemart_manufacturers_en_gb');
        $db->setQuery($query);
     $items = $db->loadObjectList();
         $options = array();
         if ($items)
         {
          foreach($items as $item)
          {
            $options[] = JHtml::_('select.option', $item->virtuemart_manufacturer_id , $item->mf_name,$item->slug);
          }
         }
         $options = array_merge(parent::getOptions(), $options);
         JHtml::_('script', JUri::root() . 'modules/mod_mymodule/js/js.js');
         return $options;

    }


}

countermanu.php (text field of counter)


<?php

defined('JPATH_PLATFORM') or die;
JFormHelper::loadFieldClass('Text');
class JFormFieldManulist extends JFormFieldText
{

    protected $type = 'Countermanu';


    protected function getOptions()
    {

    $doc = JDocument::getDocument();
       $js = '
              jQuery(document).ready(function($){
                  var count = $("#jform_params_foreignmanuf :selected").length;
                  return count;
              });
       ';
       $doc->addScriptDeclaration($js);
    JHtml::_('script', JUri::root() . 'modules/mod_mymodule/js/js.js');

    }


}