Those labels exist, at least in Japan. They're not numbers, though. http://www.barcode-net.com/products/recycle/img/ex-img_recyc...

The problem is when items have multiple labels, because they're not made of one thing. The simplest example is Saran wrap, where the box is "paper", and the part to cut plastic is either "aluminium" or "plastic" (there are brands that make it in paper so that it's easier, though). And both parts go in different bins. Bottles can have 3 parts: plastic cap, PET bottle, paper label. All in different bins.

Then you add rules where things need to be clean, which mean in theory, your toothpaste tube needs to be cut open, and cleaned, before putting it in the bin.

And different cities have different rules.