Actually.. it's less of a problem of how many are logged into the server. The root of the problem was instanced content and duty finder overload. SE is capping how many can be logged in at once to keep those subsystems from getting overloaded. We were fine with a heck of a lot more players just being in the game--it was when they joined specific content that overstressed these secondary systems that problems arose and people got locked out with their characters "still logged in" and such. SE is now choking down player access to prevent an overload on those servers, because they are linking data across multiple worlds and can't handle that combined load---the individual world servers can handle more players so long as they aren't hammering those subsystems.

Part of their plan tomorrow is to introduce more duty servers and break things down to run in smaller clusters per duty server to (hopefully) resolve this issue. If successful, it theoretically means they can increase the caps per world again. How much of an increase remains to be seen. They will likely make adjustments in stages to make sure things remain stable, so it may still be a short while before we can see log in stability like we want.