As far as Clan Vagiroving I have: Godun, Lek, Svana, and Sven. That's just according to the encyclopedia though, based on who's wearing armor.
For Clan fen Derngil it's just Ergeon and Sein I believe.
Ergeon should have a party cap of 20 (base) + 125 (clan rank 5 * 25) + 37 (stewardship 150/4) =
182 at the start of the game.
Sien on the other hand has 20 + 75 (clan rank * 15 for non-leader lords) + ~12 (his stewardship doesn't show because it's <60) =
107.
The breakdown of party size factors are:
- 20 base
- Stewardship / 4
- Clan rank * 25 for clan leaders
- Clan rank * 15 for non-clan leaders
- An extra 20 for a faction ruler
- 50 for Clan leaders only, from the Noble Retinues policy
- 80 for a faction ruler only, from Royal Guard
Then I'm not really sure if the perks are factored in yet at this point.
Clan ranks will steadily rise as the game progresses, but I think rank 6 is reserved for faction rulers only. Stewardship though, I'm not quite sure how it works. Some lords grow a lot over the span of the game, some a little, and some none at all. Doesn't seem to be linear from what I can tell.
I'm not sure what might prevent Ergeon from recruiting a full party. Money maybe I guess.