How many installations can I make with a single license?
Each license permits you to install the software once. However, you may create a single additional test installation for testing purposes only. Any test installation of this kind must be password protected, and access to it must be limited to you and your website staff.
It's quite simple, this very cheap and competitive software is affordable by consumers. And if you run two boards, you need two licenses.
What you could do perhaps is point the 5 domains to one unique single installed instance. So it has the same main url, the same header, users, and content. But the other 4 domains alias to it (location bar rewrites it to that one main one).
And then because of user contributions or ad-income you can invest in additional licenses and grow the network where those alias domains become their own.
Create a business model for your site, where buying 5 licenses up front is an investment you can earn back within reasonable time.
At least since the stable of 1.x there is an option for bulk pricing. And a generous one that is.
I'd just consider it the cost of doing business. We don't know what the "teams" fenx mentioned are though if they are professional teams of some sort then it's just a business expense. If they are under-water basket-weaving and speed knitting teams then it may be time for an internal fund raiser to get the case. Then there are always non-commercial forum alternatives.
I just didn't follow the "I am only using it on one machine" line of thought.