You are only REQUIRED to install the AnyConnect Apex license on the ASA if the remote access VPN is using any Apex-related features. If your Apex feature use is limited to ISE Posture then the ASA will work fine with just Plus licenses (even though you are required to have the Apex licenses in the sense of the overall deployment).
That said, if you have them go ahead and install the activation key(s). It's one less thing to worry about later during some 3 a.m. troubleshooting call when something isn't working and whoever is looking at the problem wonders why the discrepancy.
Thank you Thomas,
That link is really helpful. Can i directly upgrade ASA from 9.12 to 9.8
This guide says yes, but it's a bit confusing..