Jun 03 2021 01:16 PM - edited Jun 03 2021 01:17 PM
Excel only calculates with numbers up to 15 digits. Better said, numbers with more than 15 digits are rounded to 15 digits.
If this is a credit card number, then you probably do not need to make calculations with it. If that's the case, make sure to format the cell as Text before you type the number in it. This way you will get all the 16 digits in it.