How many interfaces does pfSense support?

From PFSenseDocs

There is no limit we are aware of. There are multiple installations with more than 50 interfaces (VLANs). At this time, once you get up to that many interfaces, parts of the GUI don't function completely properly (tabs scroll off the right side of the screen, themes with the drop down interface menu may not be compatible, etc.).

There are plans for pfSense 2.0 to improve how VLAN interfaces are handled to avoid these issues (which are mostly an annoyance).

OS Issues

Most hardware should not have trouble and will accommodate as many interfaces as you can fit into the box. FreeBSD occasionally has trouble when you put 6 or more physical interfaces in a system. Some systems won't detect more than a certain number of ports with certain cards. When cards share an IRQ with other devices, they may be unreliable or not work at all.