a. Assuming the listing screen for a Business Seller is similar to mine, under the price box, Number of items. I do not think you can do this for auctions only BINs.
b. I really have no idea, obviously if you had 5 and one sold it would show 4, but think it would be up to you to update any additions.
Only by making number of items more than 1 would you see any options available..