You can override the method Mage_Sales_Model_Quote::validateMinimumAmount. This doesn't allow customers to checkout if the order is under a certain amount. You can add your own condition to disallow checkout for orders with less than X products.
Use this if you want the total qty of products to be at least 10.
$minQty = 10;
if ($this->getItemsQty() < $minQty){ //$this is instance of Mage_Sales_Model_Quote
return false;
}
or this if you want at least 10 different products in the cart
$minQty = 10;
if ($this->getItemsCount() < $minQty){ //$this is instance of Mage_Sales_Model_Quote
return false;
}
validateMinimumAmount()is checked while the product is added to the cart. I would need to check the cart contents on/checkout. – PiTheNumber Sep 23 '13 at 14:42