It wouldn't be too hard to limit the number of registered users that are online at a given time, basically 2 places in ubbt.inc.php (the do_login function and also in the authenticate function where it checks if the remember me key is set. In those 2 spots you'd need to do a COUNT of the total # of users already in the w3t_Online table, before actually inserting/updating their session id in the Users table.
However, the main obstable is the fact that guests don't go through any type of login procedure. So there really wouldn't be any way to limit users that don't login. So, you might have a total of 400 users online, and someone tries to login and they'd get the error message while anyone browsing while not being logged in would not. This kinda defeats the purpose.
The other way to accomplish this would be to do a count of the online total on every call to authenticate. The problem with this method is it would just give the errors out to random users, so a user might be logged in and then all of the sudden get an error that says, sorry we're full. Again, this kind of defeats the purpse and would get rather frustrating for the end user. Not to mention the extra query done on every page to see how many users online in total.
These are the primary reasons why this isn't a standard feature. Of course, maybe someone can think of some other way around the issues above