I made a listing and had two of the same item, so I put in quantity of 2 in the draft. Then, I decided that the second item (these are handmade), was sufficiently different that it needed its own listing, so I copied over the 1st one, and made small changes to the text and changed the photos. But when I went to change the quantity number from 2 back to 1 for each listing, it wouldn't 'stick.' It showed it as being changed when I entered the number, but when I saved the draft and went to my listings, both showed that there were 2 of the same. I am working in Chrome.
I believe you need to publish and then go in and change the quantity. There are just some things that can't be done un draft mode.