Number of Items Based Discount extension.Discount can be set value or percentage.
Quantity Based Discount

Demo Links: Front Demo and Admin Demo(demo/demo)

How it works
Copy the contents of the "upload" folder to your Opencart store's top level directory, preserving the directory structure of the upload folder.
If using OpenCart 2.3 or later, you can install the extension by using the
opencart Extension Installer. First, you have to enter the correct FTP
information for your site in the System > Settings > FTP area. Then, browse to
Extensions > Extension Installer, click "Upload", and select the .zip file for
the extension.
1. Log in to Backend
2. Go to Extensions->Order Totals->Number of Item Based Discount
3. Install the extension and click on Edit link
4. You can set discount based on the number of items in the cart
6. you can check discount on front-end cart page

Features
1. You can define Number of Item-Based Discounts
2. You can add Discount to the number of items in the cart
3. You can set discount as value or percentage
4. It will not overwrite any of your opencart files
5. easy to use