I'm just wondering why there is a limit in all Cisco ASA models when it comes to limiting the number of AAA server groups to only 16? I guess it shouldn't be so difficult to allow the ASA OS to allow more AAA server groups and servers per device? Is this just because of marketing reasons or what? :)
An enhancement request was made to increase this value.
If you open a tac case and ask to have it attached to the bug:
CSCsh23977 Capability for more than 15 AAA server groups on the ASA
That will put more weight on this enhancement and it will be more likely to be addressed soon.
That should help resolve your concern.