Active Directory Federation Services can be installed in a farm. There can be maximum of 5 ADFS-servers in a farm using locally installed SQL Express. When You add in the sixth, You will have to install Full SQL-Server instance for it. If You wanto to Loadbalance the farm with ex. HA-Proxy you will need to add a proble for port 80. Using the url below. http://INTERNAL_FQDN_OF_ADFS_Server/adfs/probe or ip-address. Notice that it's http not https. ADFS add bindings for both at the time of install. When someone inside the farm fails to receive traffic. HA-Proxy will redirect to working servers. Ofcourse You can install ADFS Proxy for this purpose., but still HA-Proxy works nicely. PrimaryServer doesn't switch automatically, so You have to do it manually with Powershell. On the new PrimaryServer: 
Set-AdfsSyncProperties -Role PrimaryComputer And on all the others: Set-AdfsSyncProperties -Role SecondaryComputer -PrimaryComputerName <internal_FQDN_of_the_new_Primary_Server>

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