According to international standards, the length of credit card number is between 13 digits and 19 digits. Maybe the number of card numbers varies from region to region. Take China as an example. Generally speaking, most credit cards are 16 digits. Among them, the number 1 to 6 represents the credit card issuing bank, the number 7 to 12 represents the serial number of the user card, the number 13 can distinguish the main card from the auxiliary card, the number 0 represents the main card, the number 1 to 9 represents the auxiliary card, and the number 14 represents the issuing date of the card statement.