Hi Andre,
Yes. In an High Availability setup, while AP balancing may be distributed based on HA mode (Active-Active, Active-Passive, Site-Assigned) each controller is engineered to be resilient to the whole capacity of the pair. I.e In the case of Failover event (Controller shutdown/upgrade for example), the peer controller will support the entire capacity of the pair. In the case of a pair of E2122, the 'surviving' member would indeed support up to 4000 APs and 32,000 end-user sessions.