Q: I'm using Hyper-V Replica to replicate virtual machines. What licenses do I need on the replica server?
A: Hyper-V Replica provides asynchronous replication of virtual machines running on Windows Server 2012 and later Hyper-V servers to another Hyper-V server. The virtual machine on the replica Hyper-V host is in an off state and is turned on only in the event of a failover. Many organizations wonder about the licensing required for the replica virtual machines because they aren't actually running on the replica Hyper-V server.
The answer actually depends on the licensing you have on the primary site. If you have Software Assurance for your primary site (and the virtual machine operating system instances) then you can have cold disaster recovery (which is what Hyper-V Replica counts as) without any additional licensing required. This is documented on Microsoft's "Software Assurance by product" page. If you don't have Software Assurance for your primary site, then you would need licenses for your disaster recovery (replica) virtual machines.