Credit Card Numbers Meaning
1st Number: Known as the Major Industry identifier (MII), the first digit of a credit card number tells you what type of company a card is affiliated with. This could be an airline, a banking institution or a retailer, for example. It also helps indicate which major card network the card belongs to: American Express(3), Visa (4), Mastercard (5) or Discover (6).
Numbers 2-6: Together with the first digit, these numbers represent the Issuer Identification Number (IIN), also called the Bank Identification Number (BIN). This clarifies which credit card company a card comes from, which isn’t always obvious.
Numbers 7-18*: The seventh and all subsequent digits in a card number, save for the last, identify the individual account in question. You will only see a few of these digits printed on your monthly statement. There are 1 trillion possible account numbers for each credit card issuer, according to Discover.
Final Number: The last digit serves as a final check for payment processors. It’s basically part of a math trick to verify that a card number is genuine.
https://wallethub.com/edu/cc/what-is-a-credit-card-number/44066
I would guess that the sites that have your card number stored would only check numbers 1-6 to validate that it's a potentially viable card number.
The last set of numbers would only be used to charge the account when you try to check out and pay so I think you could make up these numbers along with the 3 digit CSV number on the back of your credit card but I'm not a banking expert.
