I have seen systems where you can dynamically generate virtual credit cards, I think which is described here. Wondering how this works, how do you generate a valid virtual card. Not which service to use, but how those services do it internally.
I use Citi which randomly generates numbers to produce a Virtual Account Number. It's a temporary 16-digit account number which has its own expiration date, security code, and a dollar limit (chosen by the user). Usage is billed on your regular monthly credit card statement.
It would stand to reason that they have a large pool of unused numbers available to be used temporarily. When in usage, they would be tagged to the primary credit card. Once expired, back into the pool for possible future use.