Hi, usually you need to license all sockets for every Hyper-V you are going to perform backup from. So, if you are planning to perform backups when VMs are running on another host, then you need to have it licensed as well. Technically, you can revoke licenses from the host that is down manually after each failover - in that case, the licenses will be reused for another host when the job runs. But this is something that you would have to be doing manually every time VMs switch the hosts. Thanks.