Microsoft CAL

I’m new to this licensing, so please bear with.
I have a win2000 server as domain controller, for 80 users, and 2 new server 2003 member servers. I have 40 CALS for the server2003 servers. Not all users access the Apps running on the 2 new servers. I need to replace the existing domain controller, but I am confused by how many new CALs I will need for this new server. Do I need to purchase 80 CALS for the new server, or just 40 CALS (80 total – 40 owned)?