How DRKs function in Ishgardian society is simple - as the job description says, we're lawless vigilantes who excise the corrupt parts of the Ishgardian theocracy by any means necessary. To be a dark knight is to suffer - you're persecuted for what you do, for what you are, but nobody else will take up your burden and right the wrongs you do because of that persecution. If you succumb to the darkness, you could very well become what you fight and end up on the end of another dark knight's blade. Dark knights have no illusions about what they do - they're criminals and murderers, but they commit the crimes and kill the people they do for the greater good.

To be a dark knight is to walk a very, very fine line between a merciless guardian and a common criminal.

Anyway, dark knights don't need an organization (Brotherhood of Darkness anyone?) because having a hierarchy or something like that undercuts one of the central ideas of being a dark knight - enforcing your own justice. If we answered to someone higher up, we'd just be taking their orders, not following our hearts. Having a central organization also implies having an emblem that would make you instantly recognizable to your enemies, something no dark knight needs to deal with.

Along with Sidurgu, are we the last dark knights?

Mmmmmmmmmaybe... any vigilante can pick up a big-ass sword and call themselves a dark knight, but without the void arts we wield they're not a dark knight proper. That said, even if the dark knights of Ishgard are almost nonexistent, there could be dark knights (or something like them) in future places.