How many interfaces does pfSense support
There is no limit we are aware of. There are multiple installations with more than 50 interfaces (VLANs). If there are many interface tabs, a drop-down list is presented. There may be some quirks in the GUI with certain themes, and some areas may take longer to display and process, but it does function. We have received reports of systems with thousands of interfaces that are still usable.
Most hardware should not have trouble and will accommodate as many interfaces as can fit into the case. Historical issues with multiple cards largely do not exist any longer, but may vary from driver to driver. With a large number of physical interfaces, the number of mbufs will need to be increased, sometimes very high. See Tuning and Troubleshooting Network Cards.