Q: Stop order if cart quantity is more than All,
I need to solve this situation:
If my magento cart total quantity products is more than 5, then I need to stop the order process and activate something like "call for price", because if you order more than 5 products I need to find the real shipment costs that magento can't do.
There is something I can do for this?
I have found something like this http://www.magentocommerce.com/boards/viewthread/47362/
Can I implement that?
Some modules?

A: I'd recommend using the event/observer system in Magento. Inchoo has a post on how to use this to limit orders over a certain cost.
http://inchoo.net/ecommerce/magento/magento-maximum-allowed-order-amount/
You could do something very similar but check for the number of items in the basket instead.

A: You need to use Magento event/observer pattern. In your case you can use sales_order_place_before
