The site license covers one server and up to 25 clients.
As for the domain issue, what has been reported is that clients that are domain joined have been unable to activate the regular retail subscription (3 PCs in a Circle for each LiveID) and the activation process on that PC directs the user to the OneCare for Server activation path. You can install OneCare for Server on Windows Server 2008 64 bit, but the clients are not covered. For that kind of an environment, I'd suggest looking at Forefront Client Security (as well as Forefront for the server).
-steve