Why not allow each clan a set number of parties, based on level like the player's. But add more people to the clan. whoever they maybe, companions, cousins, family members, distant relative ( that are summoned to be in charge of new parties, that don't necessarily appear in the clan display before). Or in worst case scenario, give the party's size to other clan leaders based on how close in the friend list they are. When people from the ai clan is taken prisoner, one or two parties will drop off some number and go on rescue missions (so they're less likely to get caught), or they can make it so the AIs do prisoner barters too. The prisoner self-made escape chance should be incredibly low