If you want to generate your own serial number for the Customers table while displaying the records, use the ROW_NUMBER(). As mentioned in BOL "ROW_NUMBER() returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition"
Here is a query that will help you generate your own serial numbers for the CustomerID
SELECT (ROW_NUMBER() OVER ( ORDER BY CustomerID)) AS ID ,
CustomerID, ContactName, CompanyName, ContactTitle