The latest information on Castles Brian situated on Terra Firma comes from Shattered Fortress, pg. 65 where only a single complex is mentioned. But I'm erring on the side of caution to get a definitive answer.
The Sarna Wiki mentions that Terra Firma has five Castles Brian and references Historical Liberation of Terra: Volume II as a source. However, after combing through Volume II and I, I couldn't find evidence of a set number of Castles Brian located on world.
I suspect whoever edited that information may have mistaken Tyrfing, which is listed as having five Castles Brians, for Terra Firma. So, is it safe to assume that Terra Firma has one Castles Brian, or is there another source to look to?