The Pro-C controller is supplied with a factory-installed base module for up to 4 stations on the PC-400 and up to 3 stations on the older PC-300. Additional modules may be added in increments of 3 stations (PCM-300) or a single 9-station (PCM-900) to expand the controller’s station capability to 15-16 stations.

After inserting station modules, pressing the reset button should be done to allow full recognition.

https://www.hunterindustries.com/support/pro-c-module-compatibility