Imo, the race admin should simply judge the situation. If 3 cars crash but are clearly not going to cause any damage to anyone else and are moving towards the pits there is no reason to call a caution really...
If however 1 car spins and blows his engin on the racing line, then caution should be called.
So it should really be up to the race admin to decide depending on the situation.
Imo, 3 cars or more seems fair. But 2 or more is better because poor dude who probably got hit by accident needs a break lol