A standard Visa or Mastercard credit card number has 16 digits.
Those digits form the primary account number, or PAN, used to identify the payment card. The number is not simply the customer’s bank-account number: its opening portion identifies the issuer or card network, while later digits identify the account or card relationship.
The final digit is a check digit calculated with the Luhn algorithm. Payment systems use it to catch many accidental entry errors, such as mistyped numbers. The visible number is only one part of a card’s payment data; expiration dates, security codes, chips, and tokens serve other functions.
Sixteen digits is the normal answer for Visa and Mastercard cards, but it is not a universal rule for every payment card. Visa can issue cards with other lengths, including 13 or 19 digits, while Mastercard’s principal ranges use 16 digits.