Only one key comes with Unity 3.1 and below. Unity 4.0 uses a different licensing mechanism, but it is still bound to exactly one server. The purpose is to enforce that you buy a Unity license for every server you run it on.
For lab purposes, Unity will run without a license in "demo" mode, where you may have only a very limited number of users, and only 15 second messages, but almost all the "bells and whistles" features are enabled for you to play around with.
