You can use number of items for example 10 when they are identical items on any listing, but generally a private seller is not expected to have quantities of identical items.
Being coins I expect you mean different items, which I think you will find is intended for business sellers.
I think you would find for coins, unless job lots, easier and safer to list individually, especially as you have up to 300 free listings a month.
The link below will explain how to list with variations, but is only suitable for BUY IT NOW, and not auctions. As the value of coins can vary considerably, auctions may get far better prices than a BIN as there is no upside to your start price.
https://www.ebay.co.uk/help/selling/listings/creating-managing-listings/multiquantity-listings-listi...
