3 here... and looking seriously at buying 2 more for forums that don't even have any users yet. I'm getting tired of the lack of support from the "open source" product. Simple question about something that is purported to be supported posted almost 24 hours ago, around 100 views and no responses. Be interesting to see if it EVER gets a response.
I really don't see a point buying multiple licenses if you can't bring at least one forum to an active state with members and decent activity before moving to the second/third/whatever.
It's nice to have interests in many thing, but keep in mind that most of your interests are already integrated to bigger/successful forums, and it would be very hard to compete; especially if you try to be involved in many areas, open dozens of forums, while not putting your entire effort in one or two of them.
Unless of course if you are bored, then whatever makes you busy
From those 6 licenses, I got two big forums (over 200,000 visits per month), 2 established ones but with low participation (still with a lot of content over the years) and two that are just starting now.