You would require 2 licenses for FWC for High Availability.
Fortinet’s FortiWLC provides for hitless failover with N+1 redundancy. The optional N+1 redundancy software feature, when implemented, allows a standby N+1 controller in the same subnet to monitor and seamlessly failover more than one master controller, and are considered to be an N+1 cluster. The standby monitors the availability of all the master controllers in the cluster by receiving advertisement messages sent by the masters. If advertisements are not received, the standby changes state, assumes the IP address of the failed master, and takes over operations for the failed master. Because the standby already has a copy of the master’s latest saved configuration, all configured services continue while the controller switches from standby to active state.