You can set-up minimum stock level for your product inventory so you can be notified through e-mail when a stock is low.
To do that, you have to go to Products -> Inventory -> select the product you wish to edit -> click on Edit. Fill up the Min. Stock Level fields for each location (if you have multiple warehouses), then click on Update.
You can also turn off low stock notifications if you don't want to be notified whenever a stock goes low. It can be done on the company level or for individual users.
For individual users, go to My Account -> My Profile -> Communication Settings, then turn off Low stock notification. Click on Update once done.
For company, go to My Account -> My Company -> Notifications, then turn off Low stock notification. Click on Update once done.