There are twelve High Priestesses in total who govern all the priestesses across Prythian, many of whom reside in the Dawn Court, Day Court, and Winter Court. While the High Lords ruled Prythian from their thrones, the High Priestesses are said to reign from their altars. They oversaw the ceremonies and rituals of the fae, recorded histories and legends, and served as advisors and confidants for their lords and ladies on matters great and trivial. According to Lucien, the priestesses get their magic from their ceremonies, and can be utterly lethal should they choose to be.
The priestesses wear a shining silver circlet set with perfectly oval limpid blue Invoking Stone, although Ianthe chose to wear a plainer circlet more common among other priests with only one Invoking Stone. Their robes, like those of other priests were billowing, artfully twisted and layered robes that are pale blue-gray in color. The robes are far from matronly hugging the curves of their body. There is also a panel of cloth atop their hood normally folded under their circlet that can be pulled over the brow and eyes for when they prayed, beseeched the Cauldron or even needed to think. It was typically worn folded back.
Priestesses are allowed to dally, marry, and bear children as it would dishonor the Cauldron's gift of fertility to lock up their instincts. They have great power in the hierarchy within Prythian, and their children are typically powerful and as well-respected as any lord's child. They often choose to control males such as the High Lords through seduction and sex. When they pursue males, they are highly-ranked and possess sufficient power.
In A Court of Mist and Fury, Rhysand says that the High Priestesses have burrowed into a few of the Courts - Dawn, Day, and Winter mostly. They've entrenched themselves so thoroughly that their spies are everywhere and their followers are near-fanatic with devotion. And yet, during the fifty years of Amarantha's rule they escaped and remained hidden.