GW: "A Vicarius has a vote in the High Council when they aren't one of the Triumverate."
Me: "Okay, so how many people are there on the High Council?"
GW: "One for each Major House."
Me: "...and how many is that?"
GW: ¯\_(ツ)_/¯
THAT KIND OF MATTERS A LOT. DON'T TELL ME THAT THESE FUCKERS HAVE 1/Nth OF THE LEGISTLATIVE POWER OF TOR LITHANEL AND THEN REFUSE TO GIVE A FIGURE FOR N.
So, apparently there are 4 ruling vicarii. One of whom gets to be part of the Triumvirate as a prefect. The rest sit on the High Council.
So High Council is 3-4 vicarii from the border wards and the Major Houses from the Ward of the Sun. All of which are Toriour, so yey for classism?
Also, there's a Senate, so Mat is in for some politics.
As for the Major Houses... well, some of this is guesswork and extrapolation, but bearing that in mind:
While there are no maps of Tor Lithanel I've been able to find, there
is a fairly nice drawing of it. And while we know GW isn't a fan of things like accuracy, or scale, some extrapolation can be done from stuff like the image, the three towers and the line saying that the Silver Tower is in the middle of the capital. So from what I'm seeing, there's 4-5 Major Houses in in the Capital itself, and several Major Locations outside the capital, but still within the Ward of the Sun.
So High Council is something like 3 rulling vicarii (since 1 is prefect in Triumvirate),
5 Major Houses of Tor Lithanel,
2 Major Houses for Verdan Ithil and Verdan Lauroi,
at least one for Verdan Lithridom, but more likely 2, depending on how public the the Well is, since I doubt one house can handle the Well and all the pilgrims,
2 more for Ystin Vaul Menluith Qhaysh splitting the rituals between Vaul and the spirits.
That gets us up to 14, so if we throw in another 1-2 for the less famous/weaker Major Houses on the outskirts that gets us a reasonable supposition of around 16.
We can then cut that down, as some texts clearly show the ruling vicarii are from said Major Houses (at least some of the time, but in context, it's likely often, if not always) so the number gets cut down to an even 12.
TLDR: Depending on if the Rulers of the Four Wards of Laurelon are from the Major Houses or not, the High Council is between 12-16 members.
If you really want to slim it down, drop one each for Verdan Lithridom and Ystin Vaul Menluith Qhaysh and the outer Houses two, which would drop the minimum down to about 8-9.
5(Capital)+2+1+1=9 with a possible -1 for the Triumvirate taking one member.
Hope that helps.