It all depends on what kind of license you buy. Where I work we've purchased a VLA ( Volume License Agreement ) which means that I can install one version of FileMaker Advanced ( my machine ), 2 Servers and 10 clients.
I believe FileMaker do offer different VLA's, otherwise the licensing is one key for each machine.
Have a look at http://www.filemaker.com/purchase/store/licensing.html
Hope this will help you ;-)