Each Operating system on your network needs an OS Licensing. General, this is done when you purchase your hardware.
For example, if you purchase a dell server and a dell workstation (with OS) each would come with a license of the OS.
So, when you buy your Windows 2003 Operating System, it will come with an operating system license. However, I assume that you are going to create a domain....so you also need to own CAL's in order for you to legally use the client server networking capabilities.
To do this, you will need to buy a total of 20 CALS (if you have 20 users and 20 dekstops).
http://www.microsoft.com/windowsserver2003/howtobuy/licensing/overview.mspx