A credit card number is normally 16-digits long, although the actual length varies depending on the card company. The maximum length of any card is 19 digits, and companies follow a specific identification protocol for number length.
American Express cards contain 15 digits, and Diners Club numbers contain 14, 15 or 16 digits based on the type of card. Discover cards have 16 digits. MasterCard has 16 digits, and Visa cards are 13, 14, 15 or 16 digits long. The first digit of a card number indicates the industry. For example, 7 is for petroleum. The digits provide added security against credit card fraud.