One thing I have been thinking about recently is how many credit card numbers are available to each credit card company. For Visa and Mastercard (MC), each card is 16 numbers long. For American Express (Amex), each card is 15 numbers long. I don’t know the number of digits for Discover, so I guess it doesn’t pay to Discover–come on, you knew that one was coming. As for other card issuers, I’ll ignore them for the purposes of this exercise.
Visa’s credit cards begin with a 4, MC’s credit cards beging with a 5 and Amex begins with a 3, which means Visa and MC have 15 possible digits that can follow the 4 or 5, while Amex has 14 possible digits that can follow the 3.
For this thought exercise, I’m going to make two assumptions. First, there are 10 numbers available for each spot on the credit card (0-9). Second, each number can be used more than once in each spot. It means there are a possible 10^15 (one quadrillion) credit card numbers available for Visa and MC and 10^14 (100 trillion) credit card numbers available for Amex.
Those are a lot of credit card numbers. I guess it makes Billions and Billions seem like a drop in the bucket.