Cluster capability is driven by WiNG version used. Running code below 5.2 allows only two cluster members in active / standby mode. Running code above 5.6 allows use of up to 6 members, when only one can be active, rest is standby.
Number of adopted APs is given by single cluster member. In case of failover the next member has to be able to handle all APs. For WM3600 it is 256 managed devices using locally bridged mode or 48 using tunneled mode.
Smallest increment of licenses is 16 indeed. See a GTAC article under following link
general rule of thumb is always use active / standby setup. Certain services like Captive Portal etc. relies on a single adopter etc.
I am just looking into older release notes and here is the statement:
"Only two (2) controllers in a cluster are supported in WiNG 5.2 and higher, the same as in WiNG v5.1.x. Cluster creation changed in WiNG v5.2 as compared to WiNG v5.1.x."Please disregard my previous sentence:
"Running code below 5.2 allows only two cluster members in active / standby mode."Only after version 5.6 system allows to use up to 6 members for RFS platforms