Keep in mind that "connection" in MS's view means each connection to the "server". So, if user1 connects to a share, and then connects to the shared printer, that's two "connections". Assuming that every user did that, you could only have 5 users.
W2K Pro will severely limit your ability to use the device as a server. W2K Server has no such limitation. (there is licensing, but you can get that.)
The server edition is also configured out of the box to optimize it's performance as a server, rather than a workstation. Just a little thing.
With 12 users...I would expect you to need the real Server pretty soon, so you might as well buy it now, and only configure it once.
--Woodie