There is a way to tell how many of an item are in stock (as long as it is not a limited purchase item like the grab bag unicorns).
Put one in your cart, then change the quantity to something like 50 or 100 and click update cart. You should see a message in red at the top of the screen that says “We’re sorry. We have now only XX units of the product YY left in stock. Please review your shopping cart.”
Just don’t forget to take them back out of your cart. 😉