I always assume the witches for casting the Dark Curse had to be a lot more specific than the spell Gothel did in HH. So Gothel may have had back up witches for other spells, but she needed 8 specific ones for the Dark Curse. Like how Regina was a witch who had crushed the heart of the thing she loves most, I assumed the others had special specifics as well. Seraphina, a witch who left a toxic situation; Hedge Witch, a witch who prefers to heal than hex; Blind Witch, a witch that has eaten human flesh; Leota, a witch who has been to the other side; Gothel, a witch that was once a nymph; Drizella, a witch that has killed another witch to get what she wants; and Mystery Witch, a witch who did something. That's my personal take anyway.