Not nearly enough information. Well, no information at all really.
The number of registered users is irrelevant - what matters is how many will be active on the site at the same time (note, 'active', not just logged in). Also what will they be doing?
What sort of operating system do you intend to use and what sort of hardware arrangement?
Have you done any trials/pilots with Moodle that might give you an indication of the resources needed?