Hey Micky,
For Homogeneous cluster, Use the formula "Cluster AP count = 50% cluster capacity"
7210 controller have a capacity of 512 APs, meaning that a cluster of 2 X 7210 has a combined capacity of 1024 APs.
In order to ensure that every AP has an AAC and S-AAC with adequate capacity for all APs to failover the recommended AP load of this cluster should be half of the total cluster capacity.
So Cluster AP size is 512 APs.
For Heterogeneous Cluster, Use the formula "Cluster AP size = Lowest value of (50% cluster capacity, Worst case scenario load) "
In you example: 2 X 7210 and 1 X 7220
50% of total capacity = (512 + 512 + 1024)/2 = 1024 APs.
Worst case scenario load = (512 + 512) = 1024 APs. [Note: Worst case scenario load is the AP load handled by the remaining nodes in the cluster in the event of highest capacity cluster member going down]
So Cluster AP size is 1024 APs
Example to demonstrate worst case scenario load:
A cluster comprised of 1 X 7210, 1 X 7220, and 1 X 7240
50% of total cluster capacity = (512 + 1024 + 2048)/2 = 3584/2 = 1792 APs
The worst case scenario load = (512 + 1024) = 1536 APs
So Cluster AP size = 1536 APs
Regards,
Kapil