When you license SQL Server (of which the SQL Server Reporting Services engine is a part of) you license what is called the OSE or Operating System Environment. This is basically the OS that has SQL Server installed on it. Now, this can be the virtualization host (VMware or Hyper-V) or it can be the Windows Server (SSRS isn’t available on Linux, so we don’t have to deal with that, but if SSRS was available on Linux the rules would be the same as Windows). You can install SQL Server (or SSRS) as many times inside that OSE as you want to, but you can’t install SQL Server (or SSRS) on any other machines.
SSRS licensing isn’t too difficult to understand, relatively speaking.