A credit card number, like a phone number or even a house number, is
not a real number - these are codes or labels. You may do some simple
arithmetic on them like add 1 to a house number and you get the next
one in sequence, but would the number divided by 2 make any sense?

You have 15 digit precision in the mantissa, and can represent numbers
up to 10 E308 and down to 10 E-307 which is a very large range. If you
want 16 digit precision you have to treat the "number" as text.

Pete