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.
Choose a stable machine that is permanently on your network. Try to avoid a machine that is frequently rebooted or taken off the network. A license server does not necessarily have to be a high-end machine; reliability is far more important.
If you are exchanging messages with the server, or if you have a lot of licenses checked in and out (hundreds per second), the amount of CPU time consumed by the server may become significant. In this case, you need to ensure that the server you select will have enough CPU cycles to spare.
If you use ArcGIS on a computer that is not on a network or on a laptop that is frequently removed from the network, you can install a license manager locally on that computer. You do not need to uninstall your ArcGIS software to install the license manager.