How does ClearPass calculate TACACS license usage and will I ever exceed the TACACS license utilization?
There is no separate license for TACACS authentication against ClearPass. TACACS is part of AAA/Policy Manager license and each unique TACACS user authentication in 7 days period is counted against the Policy Manager license capacity.
As TACACS is part of Policy Manager license, so exceeding Policy Manager license capacity for 4 out of 6 months across the cluster will lock out the Policy Manager admin UI. Administrator need to take necessary action to rectify the license utilization or increase the license count as applicable and enter a new licesne in the login page to resolve the UI lock out.