ShipStation can show you alerts on the Inventory grid when your stock is low, as long as you’ve defined what "low" is!
Head to your Account Settings (the wrench icon in the upper right).
Next, select Inventory Management and then Inventory Settings from the sidebar on the left.
On this page, look for the Default Threshold setting and set your desired value.
Don't forget to Save Changes when you're done!
Click on the Products tab.
Next, select Inventory from the sidebar on the left.
Within the Inventory grid, click on a SKU to open up the Product Detail window, then click on the Inventory tab.
Find the Reorder Threshold option, and then set it to the value that makes sense for that SKU. Keep in mind that any ShipStation alerts around reorder thresholds will refer to this SKU-specific value rather than any account-wide default.