How to apply statuses only to products with specific quantity?
The module works with the Magento product attributes as a basis for forming the product collection that receives a custom stock status. In Magento, quantity is a system attribute that cannot be used in rules' conditions. Due to this, setting a status only to products that have specific quantities is not possible.
Though, the module has a feature of setting qty-based stock statuses. With a corresponding configuration, a product will have the default status and different statuses for each range. The default status is used when real quantity doesn't fall under any range. Ranges are set as a "from...to..." quantity and could be combined with the conditions (to form a separate group of products that it will apply to) or be set to all products in the store. This way, the statuses can be applied to products that have X items in stock.
To add ranges for stock statuses, navigate to Catalog → Products → Amasty Custom Stock Status → Manage Stock Status Rules. Then choose or create a rule and enable the Activate Qty Based Ranges option and set ranges.