My guess, it's like a Sith-Apprentice kind of thing. The Jedi order was plentiful, but the Sith decided to go with a "rule of two" where a master trains an apprentice in the dark side, the apprentice kills and usurps the master and gets an apprentice themselves.
I don't know the full Star Wars lore, but I believe because the Sith were malicious and power-hungry, there was a lot of in-fighting and power-struggles.
https://www.youtube.com/watch?v=PQRfPVSaAGo
Judgement Six could choose to keep it to a minimum of six in order to prevent power-struggles or fighting amongst themselves while just enough people to manage the various branches of government and control.
I see, but why 6 members/leaders though, not 5 or 7?
I thought may be at first because it's connected 666, which is famously dubbed the "number of the beast" in the Book of Revelation, representing the Antichrist, tyranny, and ultimate human imperfection (as it falls just short of the number 7, which symbolizes divine completeness) but this is only worked if they're all bad guys (which is not the case with Illuminati in Marvel Comic since they are all heroes/good people).
Or may be it represented The Star of David, which geometry inherently features 6 points and 6 distinct triangles, but that doesn't explain anything at all.
