As the game is still in a rather early stage if you look at the feature that the current version provides the average number of people online isn\'t too high. However I believe that is subject to change once the next version is out (which is expected soon(tm), they\'re currently running closed testing).
How many people a server can handle will of course depend very much on the server hardware, its connection and the server code itself (which is under development as is the client like mentioned above).
You cannot know the uncertain. It\'s highly unlikely that the developers know it themselves. But I can assure you that the aim on the average number of players per server is a lot higher than 18.
