So my experience with this in the field is that RADIUS device admin uses zero licenses. I have done a couple of 20k NAD device admin deployments and a handful of small ones, they were all a mix of RADIUS and TACACS. Not a single one of these deployments used any of the base licenses.
Maybe this is because ISE tracks active radius sessions from radius accounting start messages, and radius device admin authentication's don't typically send accounting. I took a look, and with 20k NADs, only a handful of avocent console servers send radius accounting. I still plan licensing around the approximate number of NADs that would have active sessions, and not unique admin sessions on each device.
I would say though, something official should be added to the ISE licensing guide because it comes up from time to time and it's always fuzzy.
