How to make customers add only specific amounts of products to the cart (e.g. 4, 8, 12, etc)?

How to make customers add only specific amounts of products to the cart (e.g. 4, 8, 12, etc)?

Navigate to Catalog → Products → choose the product → Quantity → Advanced Inventory and set Enable Qty Increments to Yes. Once this setting is enabled, the Qty Increment field appears, where you can specify the step of the item qty customers should add to the cart. 



For bulk product update, there's an option to set Qty Increments via Magento mass actions. For that, tick the required products on the product grid and click the Update Attributes mass action: 



The action will open the Update Attributes configuration page. To set the required value for the Qty Increments field, go to the Advanced Inventory tab. Locate the Qty Increments field, enter the desired value, and proceed to save the changes.



On the frontend, customers will receive a notification indicating that they can only purchase certain quantities of the product: