If your debit card has an issue number, it is probably on the front of the debit card to the right of the card number. If a debit card has an issue number, it will be clearly marked as such.
Issue numbers are far more common in Europe than they are in the United States, where debit cards typically do not have issue numbers. The issue number refers to the number of actual cards that the bank has issued for the particular account the debit card is tied to. Not all banks in Europe use issue numbers. If an issue number is requested as part of a debit card purchase but the card does not have one, the best choice is to enter zero as the number.