Plugin Guide - On Shopping Cart Quantity

This discount requirement can be used with any other discount type.

To use this discount rule, the discount requirement type should be configured as Must be assigned to Shopping cart quantity and need to provide values for Minimum quantity & Maximum quantity on which you want this discount rule to be applied as displayed in the image below. The minimum quantity should be always less than maximum quantity.

Once saved, the discount requirement will be added to the discount as displayed in the image below.

That's all you need to do in order to use this discount requirement in your nopCommerce store.