You only need one license server per network to manage all of your ArcGIS products.
The license server can be either a UNIX or Windows server and will manage both UNIX and Windows ArcGIS products. For example, a Windows installation of ArcGIS 8.1 can obtain a license from either a Windows or UNIX 8.x license manager.
If you intend to run the license manager on a Windows machine, you need a SentinelPro Hardware Key for that machine. You need one hardware key for each Windows license server.
UNIX license manager machines do not require a hardware key.
Once your license manager is running, any computer on the network can access the license manager to run the software applications.