How do I get warned before a product runs out of stock?

Low-stock warnings come from the Inventory module (included on higher plans):

1. Keep the product's stock in Inventory → Warehouse stock (or Store Stock), matched by SKU. The warnings watch this stock, not the Quantity field on the product form.
2. Go to Inventory → Inventory Settings, tick the low-stock email (it is off by default) and set the threshold (2 by default).

Inventory → Low Stock lists every SKU at or below the threshold. The product form's Low stock alert field does not send this email. The In-Stock Notifier lets shoppers ask to be emailed when a product is back.