Probably loads less then you would need for signage. I reckon you could get away with no redundancy if you went with a large enough base size on the grid.

You can go as far as replacing the square “pixels” with circular dots and it still reads fine, qr codes are kind of amazing.

because a qr code containing the ~15k characters of the paper would be a full back piece of tiny dots. Probably unreadable anyways.

Because you can't read the article by reading it's number, and there isn't enough room for the whole article on normally visible parts of the body (not to mention the cost of that tattoo).

The QR code would give you a way to copy/paste the id so you can actually give and, you know, read the thing.