The number of devices you can install the software depends on the number of licenses you've purchased.
If you've purchased a single license for AirParrot 3 for Mac or Windows, this is valid for one computer. The number of users on that computer is unlimited. If you've purchased for example 5 keys, you can install the app on up to five computers. Again, the number of users on a computer is unlimited.
If you have a computer that runs multiple operating systems (either by dual-booting or using virtual machine software), you will need one license for each operating system.