Why do Credit Cards have so many digits/numbers _-_-_-_-_-_?

Answer 1

The first part of the number identifies the type of card and the bank that issued it. Only the last half is unique to the individual account. Most cards have one or more check digits built in to detect typos. This limits the number of available account numbers. Less that 16 digits would not provide enough unique numbers to go around.

Answer 2

So they don't run out of numbers and have to duplicate account numbers. There are many people with current or previous accounts.

Answer 3

because they have so many accounts,that they have to do that in order to spread them out & not have to many with ### close to the same.