there are 2240 users behind Captive Portal.
and i use pfSense 2.0.3-RELEASE (amd64) on Sun x4150 machine... (2 x Intel Xenon E540 CPU, 16 GB RAM, 4 Gigabit ethernets, 5x146 GB SAS Disk RAID5)
captive portal settings are as below
freeradius authentication, disable mac filtering, disable conccurrent logins, hard timeout 7 days
It depends on the setup and how many users you have put behind it.