I have two computers that I personally use. Do I need to purchase two licenses?
No, you do not need to purchase additional licenses. The license for Balsamiq for Desktop is tied to the user and not a particular machine.
And if you have two different operating systems, that is ok too! Balsamiq for Desktop runs on Windows and Mac. System requirements can be found on our download page.
Install it on any machine you use regularly, and enjoy!