You would want to see if they plan on getting more inventory so that you can get what you paid for and fulfill the order.
If not, then you should seek a refund if all the products aren't there for the amount that is missing. You would also want to cancel the shipment/listing in seller central and relist with the proper amount. Communication is going to be key here, make sure you are talking with your supplier.
My order from my supplier was only parially fulfilled but I paid for more units, how do I proceed?