How to configure quantity rules for individual products (groups of products)?

In order to configure quantity rules for individual products (groups of products) Product Groups for Quantity Range Statuses functionality should be utilized:
1) Navigate to Stores>Configuration>Amasty Extensions>Custom Stock Status and set "Activate Product Groups for Quantity Range Statuses" setting to Yes in general tab

This will allow you to set up rules for a particular product or group.

2) Create product groups by navigating to Stores>Attributes>Product and edit "Custom Stock Status Range Product Group" attribute (custom_stock_status_qty_rule)

Add attribute options that will work as groups for quantity ranges

3) Navigate to Stores>Attributes>Product and edit "Custom Stock Status" attribute (custom_stock_status)

Open "Quantity Range Statuses" tab and here you can associate created groups with quantity ranges

4) Enable Quantity Ranges Based Stock Status and associate products with the configured product group