Assigning a number value to a record

I was wondering if someone could help me with a database that I am working on. I am using Access 2003.When an employee joins the company they need to log hours for every day until they leave at the end of the contract. I would like to assign no1 to their first shift and count until the first day they did not work and then begin the count again. The purpose of this is to ensure they do not work more than 13 shifts concurrently. Can anyone help
Thanks