Please ask your supplier to produce the cards with UNIQUE SEQUENTIAL QR/2D barcodes on each card. It is best to place the barcode on the centre of the card and make it as large as possible (obviously leaving sufficient space for any branding you would want to include).
We would recommend using 5 digit numeric codes:
10001, 10002, 10003, 10004 etc.
See below some examples we have had done recently