Buy Now allows the seller to specify a fixed price for which they’re willing to sell the item right away (separate from the start and reserve prices). You can use a Buy Now on an auction before the reserve is met.
A Buy Now price can only be added before the reserve has been met on an auction.
If a seller has more than one of the same item for sale, they add the quantity to their listing. These listings are Buy Now only, and bidding isn't an option. You can purchase one or more of the item by entering a number in the ‘Quantity’ box on the listing page.