How many digits are in a standard Visa or Mastercard credit card number?

How to get close to this number

Recall the visual rhythm rather than the number. A card number is printed in four groups of four, and that grouping exists precisely to make a long string readable and dictatable. Reconstructing from the layout you have seen a thousand times is faster and more reliable than trying to remember a count.

The exception in the explanation is the practically important part: one major network uses a different length, printed in different groups. That is why badly built payment forms reject perfectly valid cards — a developer who hardcoded a single length created a bug that only affects customers of one issuer, which is the kind of failure that goes unnoticed for years.

The number is also not arbitrary. Under the international standard the leading digits identify the issuer, a block in the middle identifies the account, and the final digit is a check digit computed from the rest — which is how a typo can be caught before a transaction is ever attempted. The length is a consequence of that structure rather than a choice.