It will depend almost exclusively on what the server is being used for. There isn't really a hard limit for the number of users, it would be more of a limit based on usage.
What features will be in use in your environment? Processing mail can be very resource intensive, depending on where in the mail flow the server is, or if it is out of the mail flow (there is a mode for that), and where encryption is being processed, on the client or server side. If you process a lot of encrypted mail, and use the Web Email Protection feature as well, you might not be able to handle as many users.
On the other hand, if all you are doing is drive encryption, one server may be able to handle 100k clients or more. There is no hard set rule on maximum user capacity.