And where your users are stored: LDAP? SQL? on freeradius users file?
And if you need it for accounting or not.
This is from freeradius.org web page - Testimonials
Jeff Carneal - Apex Internet
Using late-2000 freeradius snapshot to authenticate ~25k ppp users. All users stored in a single file and authenticated via rlm_fastusers. Mysql accounting through direct mysql API in rlm_sql. Currently not doing simultaneous use checks, though it should be easy to implement.
So authenticating 25.000 users is no problem for freeradius and the builtin users manager.
Accounting will increase performance needs so this user is using mysql. Further mysql allows you to do more complex kinds of accounting checks.
So I would say that there will be no problem to use pfsense + freeradius2 package for accounting and authenticating on the same machine with less than 1.000 users - on actual server based hardware.
An Alix Board will probably not be able to handly so much users - but I do not have any exact numbers.