Every card issuer has a set prefix that belongs to them. The first four digits of any card number indicate who issued it. This applies to every kind of card from credit cards you can use anywhere but also things like branded gas station credit cards only good at that one chain, and so on.
This leaves only so many additional digits for card numbers, and from that pool of course some are active. Others have been issued to other cardholders but replaced, so those card numbers are also off the available list. Stolen card numbers are also off the avail list. The end result is that there are only so many possible unused card numbers.
It is also important to remember that not all cards are issued the same: Amex issues most of its cards itself. They have only a few prefix numbers. But card issuers like Visa and Mastercard use thousands of member banks and credit unions to issue cards and each of those issuers will usually have their own prefix numbers.
In other words, most Amex cards start with 37***. This leaves 10 digits for individual cards for the entirety of Amex customers, past, present, black listed, all of them. Amex segregates different types of cards based on the first five digits so not all combinations are possible and available to issue.
But your Chase Visa card will have 5678 and your Bank of America Visa will have 6789 (not their real numbers) which are unique to each bank. This means EACH of these banks has 11 digits they can use even if the other banks also use the same 11 digits for a card. It won't matter because the prefix is different.
Amex and the other banks can have more than one prefix. There are public lists of which bank is which.