Some options for getting more NICs on a system with small number of PCI slots:
You can probably pick up a couple of reasonable quality 10/100 multi-port NICs on eBay. (These would have two or more ports per PCI slot). If you are looking for something "new", some possibilities include NICS described on http://www.soekris.com/lan16x1.htm and various Intel multiport NICs.
You can use a VLAN capable switch to multiplex many switch ports onto a single LAN connection to a pfSense box.
FreeBSD supports some USB NICs.