Smaller areas (for example gates and sewers) that can be captured and held by a faction for a much smaller bonus (no patrolling guards etc), but the main point here is that;
Instead of a minimum number of players required, there is a maximum limit to how many can participate in these smaller battles.
Say the maximum is 6 players to "take control" of the south east sewer exit (not exactly a glamorous location to take, but useful in ways surely). If 7 or more players of a faction are in the area they cannot start the ZC until enough leave that only 6 remain, once started only 6 members of opposing factions could enter also, all others would not be able to enter the grid while the fight is on.
Now to combat larger teams just camping the exit with a dozen more fighters, these battles take place on world map exit grids, so the winning team can choose to leave immediately after the battle. To avoid grid shenanigans in battle anyone who leaves while the ZC is on, will not be allowed re-entry, but will still count as an entrant for that factions maximum. to combat waves of "reinforcements" waiting to enter when the battle ends, a 1-2 minute delay before the ZC actually ends could occur, so new players can't just come in and clean up the victors.
So it is however many teams, of somewhat controlled size, fighting to the death or until one party flees. Different areas could have different maximums to give some variation (and even variation in the odds, perhaps a party taking an opposing factions gate has a maximum of 8, while that grids faction can bring 10 etc). Smaller teams can use fights like these to hone their skills and tactics without fear of being completely swarmed, It will also give smaller groups something to do when the larger numbers are asleep.
TL/DR? Is it possible to add ZCs with a maximum number of entrants per faction, alongside the larger, minimum entrants ZCs.