To set up a fixed order quantity for a product in Magento 2, log in as the admin and open the product for which you want to set up a fixed order quantity.

The following steps are:

1. Click on 'Catalog' and then click on 'Products'.

2. Click (or search by keyword) the product you wish to customize. 

3.  Click on 'Advanced Inventory' to proceed.

4. Set 'Enable Qty Increments' below to - Yes

You can now type in the quantity for this product in 'Qty Increments' field and then click on the 'Done' orange button.

Note: the customer will now be notified when he / she does not meet the order quantity, plus a text will be added to the product page where it is indicated.

5.  After clicking on 'Done', you will have to click on the 'Save' button to save your changes and then you will be done!