>>281709728
>Why did Yoru suddenly need an army of devils?
I think that's just how the War Devil operates. She never looked comfortable doing things on her own, and was better fit for barking orders at other people. And she was shown lamenting the loss of her comrades on several occasions.
It wouldn't be out of character for her to surround herself with subordinates.
Other Devils would also find her easier to follow than Death/Makima because Yoru shares their passion for violence, chaos and destruction.