How many passive fail-over servers can I run when licensing an active SQL Server?

Primary server licenses include support for one secondary server only, and any additional secondary servers must be licensed for SQL Server. Note: The rights to run a passive instance of SQL Server for failover support are not transferable to other licensed servers for purposes of providing multiple passive secondary servers to a single primary server. i.e. it’s a 1:1 relationship. So if you have 1 Active instance, and 2 Passive nodes, only the first passive server is considered covered as a free license. See diagram below: