In Cisco WLC release 8.9, the controller software can scale up to 20 WLCs and 3000 APs within a single RF group. This capability allows the Radio Resource Management (RRM) algorithms to function effectively across a larger network, optimizing the wireless experience by managing the RF environment and resources across multiple controllers and access points.
References: CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide Premium Edition and Practice.