I've tried with 40.000 users.
Scenario: 2 computers
pfSense 1: old VIA Nehemia CPU with Captive Portal, 2.0.1 Release (i386), 1Gb RAM
pfSense 2: Intel(R) Core(TM)2 Duo CPU P8600 @ 2.40GHz with FreeRadius2, 2.0.1 Release (i386), 4 Gb RAM
In PC 1 I have a Captive Portal with Radius Authentication over second PC.
Provisioning users to Radius has been via .xml configuration files.
I've been uploading users from 10.000 to 10.000. All changes and problems had been at FreeRADIUS computer.
Until 20.000 users, system works fine, nothing changes (well, a little bit slow)
At 30.000 users, system has been slowed seriously: more than 5 minutes to boot, load, save configuration o any other work that involves a write to disk.
At 40.000 users, system has been slowed DRAMATICALLY: more than 45 minutes to boot, load, save configuration, etc...
But, it works !!!!
I can authenticate in my CP as fewer users load.