Currently, the inventory data is not supplied to the uStore NG theme, so the value is not available to display.
However, I have some good news for you. v12.1 uStore will have this.
OOTB it is not displayed on the product thumbnail - it is shown on the finalize step where you select the order quantity of the inventory item.
That said, since the data is available in the v12.1 theme, you should be able to modify a theme with react to add the value to the product thumbnail description...