Alright, let's just go with what we know for sure:
1. There can be up to 6 sirens alive at any one time. 2. Lilith is a siren. 3. Maya is a siren. 4. Angel was a siren.
Then there is speculation:
1. Steele may have been a siren. Doesn't really matter anymore, though, as she was impaled by a tentacle and then eaten by the Destroyer at the end of the first game. 2. Unsure of whether or not sirens are born sirens or turned into sirens.
As such, we can conclude that Lilith and Maya are the only two remaining confirmed sirens. Angel was a siren but then she died, so she doesn't count anymore. There are potentially four new sirens which can be introduced at a later point, but at the moment, it's just Lilith and Maya confirmed.
Are you suggesting that moon should get off his ass and write some stories? He's like the Stephen King of gamefaqs. - Sorbz
Here are the facts regarding sirens, provided in-game:
There can only be six sirens alive at one time, three are known to Jack (Angel, Lilith, Maya)
Angel and Lilith have a connection to Eridium. Maya does not seem to share this trait.
Sirens have a special "phase" power that acesses some other dimension. Lilith teleports through this dimension. Maya creates localized bubbles of this dimension. Angel shifts to interface with technology or something.
Sirens are born sirens. Not as hard fact as the above, but reasonably inferred from Angel and Maya ECHO's.
Sirens can be a conduit for eridium energy transfer, and accelarate the process of charging the Vault key.
Aside from that, everything else is speculation.
This is my impaling wall. It's where I put the bodies.
could also be they got sirenated at a young age. reminds me of buffy the vampire slayer.
maybe there are 6 sirens at one time to correspond with 6 vaults throughout the universe and something to do with all that
also the ending pointed to that this might be the end of exploring pandora
I'm pretty sure Lilith says that that map showed 100s of vaults on different planets.
As for the sirens being born or made:
Sirens are born, not made. The tattoos are a side-effect of being a siren. According to an echo recording left by Handsome Jack only six sirens can exist in the universe at any given time. Sirens thrive on Eridium ore, a mineral that became abundant on Pandora after the events of the original Borderlands. Patricia Tannis suggests that, because of their relationship with eridium, there should be a connection between Sirens, the Vaults , and the Eridians . (Note that this connection is not between Eridium and the individual Siren, as Angel mentions that Maya has no previous connection to Eridium in ECHO recordings) Siren powers are enhanced through the use of Eridium this is seen at several points in Borderlands 2; where Lilith used eridium to enhance her phasewalk ability. This is also seen when Handsome Jack uses Eridium to increase Angel's powers. All Sirens seem to have a pair of "wings," usually made of some sort of energy. It is currently unknown why this is, though most Guardians encountered seem to have wings, as well as The Warrior. The presence of wings could lead back to the anatomy of the Eridians.
Every time I try to go where I really wanna be it's already where I am, 'cuz I'm already there
i think you are over thinking it jack says only 6 sirens can exist in the universe at one time. its likely that each game profile of borderlands is classed as a universe and our characters can cross between universes the max amount of players is 4 so we can have 4 player sirens along with Lilith and Angel. i dont think there is any more to it than that.