There are only 6 sirens in the universe because there are only 6 living Eridians in the universe.
I hope that sentence made a lot of sense.
The Eridians could have the power to transit their own powers into other beings. Seems they prefer humans since, all sirens are human(ish) as far as we’ve seen.
So, 6 Eridians, 6 sirens.
OR… let’s say there are more than 6 living Eridians at this moment. Maybe this is the power of a more powerful type of Eridian. Tannis did say they lived under a hierarchy. Perhaps an elite force tasked with maintaining a steady amount of god-like human counterparts.
Also in BLTP, it’s hinted that Eridians are crazy advanced. It’s possible.
I’m just fascinated by sirens right now, and, this is the most plausible theory I’ve ever come across on the internet. Created by myself, no less.