An Originae is a leader of an Aeternal faction. Just as there are many factions, there are many Originae, and many factions have two or more Originae. It all depends on how many Aeternalae start the faction.
The term Originae is often confused with the term Etheriginae, for similar reasons that the terms Aeternalae and Athernal get confused: Many races that are invaded by Aeternal Etheriginae who also happen to be Originae. Thus they state that they are Originae, and often people get the two terms mixed up as a result.