It would have helped to check your order and tell us exactly what quantity your order stated. If the invoice was not for the number ordered you would need to refer this to eBay. However, entering the wrong number is a common mistake and if this is what happened it may come down to you to resolve it with the seller.
Hopefully the seller will be willing to cancel (and if you've already paid) refund 15 of these items.
If any problems, as regard to to your rights eBay doesn't enable buyers to cancel purchases, only to request the seller to cancel. The fact that the seller had 16 items suggests that they're a business seller. If so, if they are registered in the UK they are required by law to accept cancellations and returns - and by eBay to accept returns. So even if they were to insist on sending these items, you would be within your rights to insist on returning 15 of them.
I hope it isn't a seller in China as they can be very uncooperative over cancellation requests.
Good luck. I hope it all works out but if you run into problems let us have more information and someone here may be able to give you more advice.
