Now I'm assuming this Councel of Mages is the Council of Archmagisters referred to in the Origins of the Mages Guild.
I must respectfully disagree with the great Proweler on this one. http://www.uesp.net/wiki/Oblivion:Caranya of the Cyrodiil Mages Guild is a "http://www.uesp.net/wiki/Oblivion:Mages_Guild", which is one rank below an http://www.uesp.net/wiki/Oblivion:Mages_Guild. I do not accept that guild members not listed as Arch-Mages can be on a council of Arch-Mages or Archmagisters.
How many Arch Mages are there? One for every Province?
The number of Arch-Mages has been a point of great debate for some time now as can be seen http://www.gamesas.com/bgsforums/index.php?showtopic=624519&hl=. It is known, however, that http://www.uesp.net/wiki/Morrowind:Trebonius_Artorius was the Arch-Mage of Vvardenfell only. There was, at the same time, an Arch-Mage of Almalexia. Upon the Nerevarine making his report about "The Disappearance of The Dwarves", Trebonius has this to say:
"Well, I'm glad that the mystery is solved at last. Next time the Arch-Mage of Almalexia visits, I'll really have something to brag about." Since Morrowind is one province composed of six Imperial districts (See http://www.imperial-library.info/savant/), and at that time there were two Arch-Mages for one single district, I would have to conclude that the one Arch-Mage per province theory is rendered unfeasible. (Not to mention that there are more than six provinces, which would put a strain on the "Council of Six Archmagisters" rule.)
That's part of the problem.
In Morrowind Trebonius wasn't "The Arch-Mage", he was an Arch-Mage and one of many. While Oblivion has a council of mages, Traven is the only Arch-Mage.
The only recourse that I can find is that the http://www.imperial-library.info/mwbooks/mages_charter.shtml reads differently than http://www.imperial-library.info/obbooks/mages_guild_charter.shtml. In that the Mages Guild for the Iliac Bay regions have http://www.uesp.net/wiki/Daggerfall:Magic_and_Spells(i.e., "thaumaturgy") and the Divines' Chapels are called "http://www.uesp.net/wiki/Daggerfall:Akatosh", "http://www.uesp.net/wiki/Daggerfall:Zenithar", etc., one could assume that each guild has the ability to set its own local rules and policies, with Traven's ban being only enforceable in Cyrodiil.
This does not answer the questions...
(1) How many Arch-Mages are there?
(2) What power does each Arch-Mage have over the affairs of the entire Guild as an institution?
(3) Is each member that is ranked as an "Arch-Mage" automatically a member of the "Council of Arch-Mages" that we keep hearing about?
(4) Are the six Arch-Mages that sit on the "Council of Arch-Mages" of a different type than the Arch-Mages that run the local guilds? For example, federal U.S. judges often have the power to overrule state judges (on certain things) even though they are both "judges".
What about Ocato? Didn't he have it, till he stepped down, and it got passed to Traven?
You are thinking of http://www.imperial-library.info/fsg/sloadarticle1.shtml by The Sload. While it is a wonderful article, it is not canon.
The Sload are necromancers and worship Mannimarco.
Not necessarily. The Sload known as N'Gasta worshipped Clavicus Vile as shown http://www.imperial-library.info/tsorg/part10.shtml.
May Julianos, as the god of sorcery, illuminate your searches, for I remain...
Yours in the Scrolls,
___The Word Merchant of Julianos