If I am reading your question right, you are asking how many users can log on to the system? That would largely depend on what else you plan on tasking the server with providing. If you are running just a domain controller, your theoretical limit would probably be between two and three thousand users, but don’t expect them to be very happy Monday morning when they all try to log in at once!
If you are asking if this is adequate hardware for 1000 or less users, I would again stipulate that as long as they aren’t all hammering your DC at once, you should be OK the majority of the time.
Two things: I would use a server with those specs for ONLY for a DC role, and I would recommend doing some research on DC roles and configure another DC. Doing so would allow you redundancy, and some FISMO optimization. There are probably several terabytes of info on the World Weird Web covering just about any DC rollout scenario you can think of, so I am not going to cover this in excrutiating detail.
Finally, if you’d like further information, please provide further detail. <grin>