I keep hearing that there is some confusion regarding the number of servers in a Lync Front-End pool and how many can be down at one time before the pool fails.
In Microsoft Failover Clusters, you need over 50%. If you have just 50%, the Failover Cluster will… you know… fail over. This tends to be what many administrators think when it comes to their Front-End pools.
Lync pools are a bit different in that regard. Please, let me be clear and say that you really should have no fewer than three servers in a Front-End pool. First, because it does allow for some additional scalability, but the main reason to have at least three is because in Lync Server 2013, Lync uses a distributed model that maintains the user’s data on three Front-End servers in the pool. By having at least three servers, you have three copies of some pretty vital data.
If the number of servers in the Front-End pool falls below 50%, the remaining Front-Ends go into survivability mode. Basically, this allows for another come back up and reestablish the 50% or more requirement. After five minutes, if the pool is still below 50%, then the remain Front-Ends will shut down.
The table shown here only shows 8 servers, but you can have more in a Lync Server 2013 pool and it will scale well.