A possible solution could be a piece of logic at the location where the available stock is updated after an order. With a split action and an expression to check is the threshold is met, you could place a microflow to send an email (or a pushnotification) to an admin. This can be done with additional modules from the Mendix Marketplace.
Or you could make a scheduled event to check each hour (?) to do a stock count and implement the same logic.
In both cases need to implement some settings, like threshold (per product?) and who to inform. This can be done with constants in the environment a settings entity in your database.