I would suggest you to achieve this from observer.

This event `checkout_cart_product_add_after` should help you.

You will need to create a simple extension to fire this event.

In your config.xml have this:

    <?xml version="1.0" encoding="UTF-8"?>
    
    <config>
        <modules>
            <NameSpace_ModuleName>
                <version>1.0.8</version>
            </NameSpace_ModuleName>
        </modules>
    
        <global>
            <models>
                <namespace>
                    <class>NameSpace_ModuleName_Model</class>
                </namespace>
            </models>
            <events>
    			<checkout_cart_product_add_after>
    				<observers>
    					<update_qty_on_product_add_to_cart>
    						<type>singleton</type>
    						<class>NameSpace_ModuleName_Model_Observer</class>
    						<method>updateQty</method>
    					</update_qty_on_product_add_to_cart>
    				</observers>
    			</checkout_cart_product_add_after>
            </events>
        ............
        </global>
    </config>


Now, your NameSpace/ModuleName/Model/Observer.php

    <?php
    class NameSpace_ModuleName_Model_Observer
    {
        public function updateQty(Varien_Event_Observer $observer)
        {
           $item = $observer->getEvent()->getQuoteItem();
              $x= $item->getQty();
              $minValue = 8;
              if ($x>= $minValue) { 
                 Mage::log('Proceed with number: '.$x, null, 'custom-log.log'); //check in var/log folder  after product adding to cart
              }
              else {
              $x = 8;
                 Mage::log('Minimum number is: '.$x, null, 'custom-log.log');
              }
              $item->setQty($x)->save();
        }
    ?>

Check if file `custom-log.log` is created in `var/log` folder after you add product to cart. If it is then your event is firing.

Hope this helps.