It is completely unknown what the eff Prime Servitor's do, it is just speculated. Archons, as we know are the Prime Servitor's Right hand dudes, so they are BFFs. The Kell are the Fallen Leaders, that is pretty much what is known. If there is more than one Archon per house, the game didn't really express it, I don't think the grimoires do either, they say Archons but I think that refers to among the fallen Archons are revered. I think there is only one though, could be more but in game only one per house so far. For example, the Archon strike they where trying to get a new Archon to join the House of Winter, because Wolves is run by the Queen. Because House of Winter Archon and Prime Servitor are missing, no idea where they are, likely will be in DLC. And I doubt we will see any more command units because it seems pretty straight forward, likely you will see just the House of Winter archon and prime servitor find some hidden power in the citadel and it causes some serious crap.