If you need a standard sequence number 1-... then you can use
ALTER TABLE tblname ADD column_name INT NOT NULL IDENTITY(1,1)
Keep in mind INT can only handle so many records, if on SQL 2K you can try bigint if needed.
Now if the item needs a specific value based on criteria you can use a trigger or build logic in procedure to pass data to and have pick the right number.
"Don't roll your eyes at me. I will tape them in place." (Teacher on Boston Public)