When stock levels are running low, sometimes we do say how many units are left. But not always. It depends on the product.
Interesting bit of info: we do NOT keep an item in reserve if you put it in your cart. Only upon submitting the order is it yours. So an item in your cart can still sell out.
This is done for a number of reasons, but what it means for you is that it's best to not wait too long before you finalise your purchase.