I am wondering how the Dark Brotherhood operated with multiple semi-independent sanctuaries across an entire continent and only 1 Night Mother, I am fairly new to Elder Scrolls lore. Were there multiple Night Mothers? Or did the sanctuaries besides the main one do something similar to what Astrid did? I can't imagine a secret organization could set up such a robust network of communication that the more rural sanctuaries could get up to date assassination orders from a continent away. Or could the Night Mother create and talk to listeners remotely?
There is only one night mother. During the times before skyrim, when the Brotherhood was still a tamriel wide organisation, there was the black hand. The black hand were like the managers of the dark broyher hood. The black hand consists out 1 listener and 4 speakers. The listener is the one who speaks to the Night mother making him second in command. The speakers ruled there own sanctuaries across tamriel, but answered to the listener. So how a contacts worked: someone performs the Black Sacrament > the night mother becomes aware of this > the night mother tells the listener > the listener orders one of the speakers > the speaker orders one of their assasins to do the job.