I remember this worked when I was in the older version(1.143 steam right now).
I used to just wait in the bandit's lair for bandits to band up.
At first they would just wander around the lair until they urgently(?) need to resupply and have to attack me and they would all join the fight when one of them attack me.
And now it seems that after waiting in the lair until the 2nd payment they wouldn't come to attack me, either their 'supply limit' has changed, or there is no limit at all.
Does anyone know a way to fix this?