theme/template/catalog/product/view/type/default.phtml

add

<?php $stock = Mage::getModel('cataloginventory/stock_item')->loadByProduct($_product);  ?>

and

  <p class="availability in-stock">
<?php echo $this->__('Availability:') ?> <span><?php echo $this->__('In stock ') ?></span>
     <?php 
         if (intval($stock->getQty())!=0){
         echo '('.intval($stock->getQty()).')';
         
         } 
        
        ?> 
    
    </p>

you will get Availability: In stock (xx)

Note: Manage Stock for products must be enabled