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)