Show amount of items you have in your barn before producing it

Hhhhhhhhhhhhh

It does show that! When you are dragging the product you want to produce, look a the TOP RIGHT corner. You'll see a barn image and a number, and that tells you how many of that product you have in storage.

edit: Sorry if you already realised this, I didn't read your reason for editing and I'm also not sure if that is why you edited it. :)