To trigger a low stock alert when the quantity falls below a certain amount, first enable the alert by going to Alerts -> Low Stock Threshold Alert/Location Based Threshold Alert.
Second, edit the Inventory or Asset Stock Details page and set the threshold amount in ‘Low Stock Threshold’ and ‘Location Threshold’ fields. The former is the total quantity of the inventory or asset stock, and the latter is for the total quantity of inventory or asset stock at any location.
Learn more: When do you receive a low threshold alert on inventory/asset stock?