The NAC Manager is where you configure all you're policies for all you're users. The Manager in turn controls all the NAC Servers within the network which enforce these policies.
Depending on you're choice of deployment(in-band/out-of-band/Virtual-Gateway/Real-IP) it is very likely that you are going to need more than 1 NAC Server to control all your network entry points effectivly.
Certain combinations are possible, you could for example control L2 LAN segments and Cisco Wireless Controllers with the same NAC server in L2 Out-of-Band Virtual Gateway mode.