There is no option for that in HikaShop. You would have to edit the file cart of the view checkout via the menu Display->Views and add some check in order to make sure that the user can only checkout when he has a total quantity divisible by 12 and if not, not display the "next" button of the checkout and display an error message.
Something like that:
Code:
$total = 0;
foreach($this->rows as $i => $row){ $total +=$row->cart_product_quantity; }
if($total%12){
$this->nextButton = false;
$app =& JFactory::getApplication();
$app->enqueueMessage('Please have in your cart a quantity of candy boxes divisible by 12');
}
