在PHP中限制多选框html

I am woking on Joomla CMS. I want to add limit on this multiple selection box using javascript please help me to do this. I am creating my list by using the following code:

<div class="controls" >
<?php 
    $attribs = "class='input-xxlarge required' name='selection[]' id='selection[]' onchange='Notify();' size='5' multiple='multiple' ";
    echo $select->getSelectCategoryTree('id_category[]', explode(',', $this->row->id_category), '', $attribs, '', true); 
?>
</div>