Okay so I like the Telvanni. I was proud to become its Archmagister. I was playing the Morrowind mod "Rise of House Telvanni" and according to its dialogue that there are three Telvanni Archmagisters (one in Vvardenfell, one on mainland and one off in some islands... somewhere) and a "Grand Archmagister" who is the ultimate head of the Telvanni. Now I understand that this is a mod and not official so I just ignored it. However! I was perusing a thread in this section and came across one post mentioning the "Mainland Archmagister" in the mod Tamriel Rebuilt.

So now I'm wondering... there are two mods that have established (unoffical) lore of there being more than one Telvanni Archmagister at any given time. So what is the reason for this? Is it mere coincidence or is there some book, source or even dialogue that hints at there being other Archmagisters in other areas of Morrowind?

The Brown Book of 3E 426 denotes the Telvanni Council in Sadrith Mora as being the council of Vvardenfell District, so we know at the very least that mainland Telvanni is somewhat seperate from the one presented in the game. If that means that they have their own Archmagister, or if the one on the island is the leader of the whole House, I can't say. It would not be unreasonable to assume, i suppose, unless there is another source that refutes that assertation. In a case like this, just think whatever makes your roleplaying a more enjoyable experience.

Can't post links, but the book can be easily found with at The Imperial Library or at the UESP.

It does seem to suggest different districts, but you are still named Hortator for all of House Telvanni if you are the Nerevarine and you're only named that if you convince the Telvanni district council. So it could really go either way, perhaps those are just the council members that live in Vvardenfell but they still make up a majority of the house in total

I think someone in-game (Caius?) says that in order to be named Hortator of a Great House, you must have the approval of all the House's council members so I assumed that all of them are living in Vvardenfell District. It's not explicitly implied in regards to the Telvanni, but IIRC, Athyn Sarethi of Redoran tells you that Bolvyn Venim had moved the entire Redoran council from the mainland to Ald'ruhn, so maybe this move was made by all the Houses with holdings on Vvardenfell, since it's the place where all the conflict and territorial disputes are happening since it was recently opened for settlement.

Morrowind's politics freely ignore the fact that Vvardenfell is really just a backwater. They don't bother to explain why you can neglect Dres and Indoril when becoming Hortator, or what exactly your rank is when you head just the Vvardenfel part of a Great House.

Most of Indoril fell on their own swords to protests the Armistice; Dres only cares if slaves are involved.

Nerevar was Hortator of those houses too, though, and you can't tell me that Telvanni gives a [censored] about all that stuff either.

I can see Hlaalu bringing Dres on board and the Temple's recognition securing Indoril, but the games just don't bother.

I can see Hortator-hood only requiring the approval of three great houses.

Of course, remember, the Incarnate's deal was the fulfillment of Vvardenfell's mythic potentiality, not mending Morrowind's political actuality.

Why just Vvardenfell? The most mythically important component I can think of is the unity of ALL the Dunmer people, settled and nomadic. The First Council. Political actualities matter for a Ruling King.

But this isn't something that breaks the main quest; it's just the prime example of the game's generally heedless Vvardenfellcentrism. Even Morrowind has a few worldbuilding flaws.

Except the actuality of the situation is that the Nerevarine isn't the Ruling King, just Uriel and Azura's (and possibly Vivec's) little time-share puppet. There's your political actuality: A good portion of the Empire's Defense budget goes towards weaponizing vague prophecies to undermine enemy god-regimes.