I think it is fine, as long as you can be comitted and spend enough time doing the necessities to keep the forum going. The problems only really arise if you fail to spend attention to a certain forum(s) which could lead to it dwindling into a pile of... rubbish, to put it nicely.
I own two and I can barely keep up with both, good thing I have my trusty sidekicks to help me out though. -8)-
I couldnt imagine owning more than that. Unless they are all inactive of course.
I'm up the three now, and I think that will be my limit. I am in complete control of two, the other I just administer and run (I don't own hosting/domain for it).
If you own more than two then you better have co-admins and proper staff. It can be hectic I should know. I owned a forum and I had wonderful staff that helped me out a lot.
In my eyes, it all depends on your personal commitments, and how much 'help' you have. For example, with Ghost Webz Network we hope to run several active forum based community's and several webpages. There's a few of us in charge though, a chain of command per say. Much like the normal chain on a forum, only extended a bit. The admins of the forum are a direct reflection of the owners of GWN, and the other GWN members are free to roam the forums as normal members, only with full admin access, and a rank allowing the other members to know (though no group color).
Since the true admins would be busy managing the database, keeping up with the scripts, banning new proxys, ect, this 'skeleton' crew would basically run the forums on a community level. There would be elected moderators to do the general running of course, but for the bigger issues this crew would do without having to track down an admin for every issue. I've developed this system after being inspired by the messages boards on GameFAQS.com where you never see any staff posting. I didn't want that, but fewer admin presence seems to provoke more posting.
I have a few forums, not all of them are active though, some though for example are small support forums I have for online services, so of course not that active