How many credit cards per person?

Another quick and fun example of everyday deceptively LARGE numbers action. I recently used my first ‘virtual’ or temporary credit card, for a USA purchase. This site generated a single-use card, pre-loaded with $n USA (but it couldn’t be recharged).  Mmm, says the mathematician in me, how many cards could each person on Earth have?

Some in the head, approximate numerical pondering followed:

(a) 16 numerical digit card number.  1o¹⁶ combinations (a 1 followed by 16 zeros)

(b) 7 billion people on Earth. 7x(10⁹).  That 7 is near 10, so I’ll round the whole thing  up to 10¹⁰

Thusly dividing (a)/(b) we can have 10 ^(16-10)  each (remember your exponential maths?). Which is 10⁶       That is 1 million cards each.