The number of devices you can install the software depends on the number of license seats you've purchased.
If you've purchased a single license for Reflector for Mac or Windows, this is valid for one computer. The number of users on that computer is unlimited. For example, If you've purchased 5 keys, you can install the application on up to five computers for the OS chosen at purchase. Again, the number of users on a computer is unlimited.
If you've purchased a universal key for Reflector, you will be able to install that key on either macOS or Windows OS.
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.