MS Licensing Question

Hi there, hope everyone’s having a good Friday. The more I read about MS licensing the more I seem to get confused… Here’s the deal though. I am adding a new server to the domain, which will be our new exchange as well. Say I have 50 users that have mailboxes. I know I need 50 cals for exchange will I also need 50 cals for the server os (w2k3 for example)? Thanks for your time and I appreciate any lincensing “translation” you can provide me.