With the release of SQL Server 2012 and specifically the AlwaysOn feature we need to rethink the way that we handle the licensing for servers that will be passive servers. In the past it’s been assumed that passive servers didn’t need to be licensed, and that was true, for the most part. The official docs read that you get one passive server per active server that you have licensed, provided that you have a current Enterprise Agreement. Without the Enterprise Agreement in place any and all passive servers need to be licensed.
With SQL Server 2012’s AlwaysOn feature we have an active server called the Primary Replica and we have up to 4 secondary replicas. Even if none of those 4 secondary replicas are in use for anything, you will still need to license two of them to be properly licensed. This is because when licensing SQL Server’s each licensed server gets you only a single free passive server. So for a 5 instance AlwaysOn Availability Group deployment you’ll need to license at least 3 of those instances which would give you two passive instances. As long as those two passive instances aren’t being used for read access they are free.