Each Contact will be tied with an external Id that is made out of a set of numbers. To keep track of these numbers, you need to create a custom field with either text or number as the field type.
If you choose to use the number as the field type, your external Id will end up like this – 1,234,567. But if you choose text as the field type, you might have bad data because it allows alphabet characters.
To overcome this, you can choose to use text field type. However, you will need some validation to avoid users from adding in alphabet value. For example: