In very poorly named thread about limits on the Quest Log over on the Official Forums, Georg Zoeller chimed in with some information about the limits (or, really, the lack thereof).
I don’t think this is a concern for us. After years of playing legit characters and testing, I don’t even know if there is a limit we are enforcing. And since you can hide quests that are not on your current planet and turn tracking on and off individually for each quest, you’re unlikely to experience significant overload either.
In a follow up question, I asked if the preferences could be set to ‘auto-hide’ quests from other planets, or if we’d have to go in and manually hide each quest.
Georg popped back in and answered with this:
That option does exist, yes.
All in all, it sounds like the Quest log/tracker system will be fairly well refined.