Credit cards usually have 16 digits but can have up to 19. According to CreditCards.com, all credit cards have common numbering schemes that signify the issuer and the user, with a check digit on the end.
CreditCards.com shows that the first six digits of a credit card are the issuer identifier, meaning the company to which the card belongs. The final digit of each credit card number, the check digit, is issued using a formula called the Luhn algorithm and is used to validate the credit card number itself. The numbering system, size, shape and material used to make credit cards come from the International Organization for Standardization.