A scarab of protection protects against 12 negative energy effects.
A shadows Str dmg atk is a negative energy effect.
A mob deals automatic dmg due to so many attackers against one defender.
So, if a mob of shadows attacks a bearer of a scarab of protection, does it drain one use from the scarab, or multiple? & if multiple, how many; 1charge per d6 dmg?
I would probably ask how many times I can use Damage Reduction against a swarm and use that as a guideline. But my intuition tells me that you're not going to to spend more than one charge per attack.
I also have a feeling that the CR of a mob of shadows would be deceptive.