Several sources point to the ritual requiring only 12 high mages:
Uaul'Selu'Keryth. In your tongue, the name might be translated as "At War with the Weave". When twelve High Mages last performed this ritual, the world was torn asunder. It is a power no mortal should possess and no god should use. (Mordenkainen's Tome of Foes)
Used only during situations most dire, twelve High Mages can harness the energy of the Weave to cause it to fluctuate across a great distance…In its ultimate form, the Weave is temporarily torn asunder, and the damage is made manifest in major cataclysms and storms, tidal waves, and permanent changes to the lands of any planet…This ritual has been invoked only once in the history of the elves on Faerûn, and the ensuing cataclysms broke up large land masses to allow the creation of the isolated island of Evermeet. (Cormanthyr: Empire of the Elves)
However, in Evermeet: Island of Elves, which I cannot quote since I don’t own the book, it is apparently described differently: there was a massive tower built, preparations took hundreds of years, and on each step of it was a mage who would take part in the casting. Neither of the previous two sources, especially not the latter (which describes in great detail the high magic rituals), mention anything more than 12 high mages.
What could be the answer to this inconsistency? Are the two sources I cited retcons? Are there actually helper mages in the ritual, only 12 of whom were actually required to be high mages to cast it?