Be sure to write the card ID number down, and/or take a clear digital picture of it, and/or send yourself that number and/or picture to yourself in an email.
Just in case you lose the card, you can still enter the ID number and get the pictures.
We have done photopass a few times and have used multiple cards on one account. Just in case you leave one card at the hotel or with another member of your group and need it, you can get another and just log in the numbers of the multiple cards when you load your photopass pics on the website.