Hi,
1. Master-local
In case you want to terminate the AP across both controllers, you can configure them in master-local mode.
However, the changes to the configuration can only be done via master. In case master controller is down, you can not make any changes to the configuration from the local controller.
As you have 50 AP's , you can have 25 AP's on master & 25 on local (you can use any ratio) & both the controllers can backup each other.
2. Master-Master /Master-Standby topology.
This requires both the controllers to be on same layer 2 network (for running vrrp).
All the AP's will be up on the master controller & other controller will act as redundant master.
In case the current master goes down, AP's will move to redundant master.
Licensing:
We can enable centralized licensing between the controllers. The number of licenses will be equal to number of AP's.
So, if you have 50 AP's , you will need 50 AP & 50 PEF licenses.
Master controller will act as licensing server. The licenses will become part of shared pool.
Eg: Lets assume all licenses are installed on the master. The 2nd controller is configured as either local/standby controller.
So, if master goes down, licenses can be used by local/standby controller as they are part of the shared pool.
Please refer to VRD or UG for additional information.