If you can want to allow interval-based quantity increments in your Shopify store, then It is possible with the  "MultiVariants - Bulk Order" app. It is available in the “Professional” package.

To get the feature, install the "MultiVariants - Bulk Order" app in your Shopify store and follow the procedure:

>> Navigate to the app admin (configure rules) and click on “Set restrictions”.

>> Scroll down and go to the section named “Quantity selection type”.

>> Click on the button “Interval” to activate the feature.

>> Set your desired quantity which you want to allow an increase or decrease under “Quantity interval”. For example, if you want the quantity interval as “6”, set “6” under the quantity interval.

>> Save and check it on the product page.