One of the simpler explanations of Server 2000 licensing is
on this web site.
Basically, you need a SERVER license for each Server. Then you need some sort of CALs. The basic Server license usually comes with five CALs included.
Once you get the Server installed, you have to choose how you are going to handle CALs. You can use either "Per Server" or "Per Seat" licensing mode.
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"Per Server" Mode:
If you have several Servers and they all get used by everyone, then you'll likely
NOT want to use "Per Server" mode. In "Per Server" mode, CALs are assigned to a particular Server. The Server counts CONCURRENT connections to itself. Individual client Computers (Devices) aren't licensed by name. You can have a thousand computers, but if only five are connected to a particular Server at any given time time, you'd only need five "Per Server" CALS for that Server. Each Server has its own CALs in "Per Server" mode.
"Per Server" mode is most useful if you only have a single Server and many client computers that only occasionally connect to it.
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"Per Seat" Mode:
In "Per Seat" mode, you have to acquire a CAL for each Device. You need a CAL for each Device that will be accessing ANY of your Servers. If a Device (computer) has a CAL, it can connect to ANY of your Servers.
"Per Seat" mode is useful if you have several Servers and your client PCs will need to access many of the Servers and keep their connections open. The actual CALs are (apparently) delivered on a floppy disk and are physically installed on the client PC.
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In your case, if you buy five copies of Server 2000, you'd likely already own twenty-five CALs. You should be able to put the Servers into "Per Seat" mode and install a Device CAL to each of twenty-five computers. Your Users will be able to access all of the Servers as much as they want from twenty five different PCs.
Remember, I am NOT a Windows licensing expert, and I've never even SEEN a boxed version of Server 2000. This is just my understanding of how it works. I deal full-time with SBS 2003 licensing, which works somewhat differently.
Note: Server 2003 and SBS 2003 have different licensing rules than those listed above.