Am I allowed to set up a test environment with the same forum license as I use in the live environment? Say, under something like www.mywebsite.com/test/? Anything special I would need to know before doing so?
A test installation is permitted under the terms of the license.
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.