Quantity drops from XX to 1 and shows as last one

[Isaac]

Hi guys,
Although I set the quantity of a listing in my seller hub as +80 , after the first sale this number drops to 1 and the item shows as the last. However, it does not end or become 0 after the each next sale. It remains on 1 after each transaction. So buyers can buy only 1, not more each time. How to fix it?

Your advice is highly appreciated!
Thanks