A night market stall has, typically, four seats and a wok. With six people you are a table. With eight you are a slightly awkward table. With twelve you are an event, and the stall stops cooking what it normally cooks and starts cooking what it can produce twelve of.
That is the moment the evening stops being a visit and starts being a performance. The food gets worse, the queue behind you gets annoyed, and the thing you came to see stops happening while you are watching it.
So the cap is eight. It costs us margin on every departure and we have never once regretted it.