Promotion event by quantity is triggered by the minimum quantity on the transaction, you can give a discount or reward gift to your customer if they buy your product with the required minimum transaction that you set before.
I. Adding Promotion By Quantity
Step 1. Go to promotion menu then click the promotion event tab and click add button

Step 2. Fill in promotion event form then click submit
General information for Promotion :

Purchase Requirement & Promotion Reward :

Step 3. Promotion event by quantity successfully added

II. Use Promotion Event By Quantity
Step 1. Go to sell menu and input the product then press the Ok button

**** Buy 2 Matcha Latte will get 1 Fresh Milk Boba
Step 2. Select reward



**** Reward options will appear if you meet the specified minimum spending requirements

You might also like to read these articles below:
To make a promotion tiered, you can check this article How to Use Promotion Tiered
To make promotion the cheapest item, you can check this article Promotion Cheapest Item
To make a promotion with minimum spending, you can check this article Promotion With Minimum Spending Get Free Gift
To make a promotion discount for specific products, you can check this article Promotion Discount For Specific Products
If you want to make a promotion discount on all product, you can check this article Promotion Discount All Product