As I understand it, the first 6 digits of a cc number are pertaining to the issuer (visa, bank etc.), which leaves 10 for actual account numbers. However restrictions are placed on certain digits by the necessity of checksums, so the range is not the full 9999999999 possible accounts, even though that number is surely passed in the number of current credit card account holders?

What will happen when all possible accounts are used, and how long until this happens?

Credit card numbers use Luhn checksums, so there’s only one check digit. That still leaves a paltry nine digits out of a sixteen digit number for the account number, which is slightly less than one billion accounts.

I imagine they recycle account numbers frequently.

You are correct that the first 6 digits identify the issuer. Most issuers have a range of 6-digit identifiers, which greatly increases the number of cards they can issue. Of the other 10 digits, 9 form the account number and the final digit is a Luhn checksum of the preceding digits. The number of cards that any given issuer can issue depends on how many 6-digit issuer IDs they have. The 9-digit account number allows for 1,000,000,000 unique accounts. Multiply that by the number of issuer IDs available to a given issuer to figure out how many cards they could issue. For example, Visa has the entire range of IDs starting with the digit 4. That’s 100,000 IDs. 100,000 IDs times 1,000,000,000 account numbers gives 100,000,000,000,000 (100 trillion) numbers for Visa cards alone. Even subtracting out numbers that they are unlikely to issue, such as 4000 0000 0000 000x, there are enough numbers available to satisfy the foreseeable need for a long, long time.

Though most credit card numbers are 16 digits long, the ISO standard allows them to be up to 19 digits long. In the unlikely case that we do run out of 16-digit numbers, they will increase the number of digits and open up a much greater range of possible numbers.

But that’s not 100,000 banks, because some of those digits are used to distinguish the specific type of card, such as credit, debit, gift, and others. How many card-issuing banks are there in the US? My curiosity is not about running out of consumer account numbers, but bank id’s.