If your debit card has an issue number, it’s probably on the front, to the right of the card number. If a debit card has an issue number, it will say so right on the card.
In Europe, issue numbers are much more common than they are in the U.S., where debit cards usually don’t have them. The issue number is the number of cards that the bank has given out for the account that the debit card is linked to. In Europe, not every bank has an issue number. If a debit card purchase asks for an issue number but the card doesn’t have one, the best thing to do is put 0 as the number.