Credit Card Authorization https://wallethub.com/edu/what-is-a-credit-card-number/44066/ 1/ Valid first number… — American Express(3), Visa (4), Mastercard (5) or Discover (6). number of digits… — American Express = 15 Visa/ Mastercard / Discover = 16 Security Code Amex = 4 Visa/ Mastercard / Discover = 3