ZenCart as of this version (1.3.8) does not have a "Quantity Discount" feature built-in. However, there is a "Quantity Discount" mod that can be installed. More information about the mod can be found on the author's website, thatsoftwareguy.com.

Now for example, if you want to give your customers 10% discount when they buy three or more items at checkout.

Turn On Quantity Discount
1. In the Admin control panel, go to "Modules -> Order Total".

2. Click on Quantity Discount and click Install.

3. In the below picture, note how we entered "3" for the "Discount Level 1" and "10" for the "Discount Amount 1".

4. Click Update button.

5. Now when customers order 3 or more items, they see...

To change or remove:
Simply go to "Modules -> Order Total", click on Quantity Discount and click Edit or Remove

Enhancements:
Now if you do set up an quantity discount, it would be nice to be able to indicate this on the front page of your store. One way to do this is to have your web developer alter the core functionality of ZenCart so that you can display a promotion message on your storefront. This is described in another tutorial.