The number in the specific URL for any Constituent record represents the System Record ID.  This may or may not match the Constituent ID in your RE database.

RE NXT URL

Open the Constituent record in the database view and the Constituent ID is a different value than that in the URL

Constituent ID

To view the System Record ID, open the Constituent record in database view and click File>Properties.  This field is also available in Query and Export, depending on your needs.

System Record ID