The Souls of The Hundred by James Palfi
In the world everyone’s soul has a certain value. There exist casters that can use magic but they are very rare 1 in a million so 8000 total.
There are two faction’s hunters and The Guild of Magic that is responsible for protecting casters. One of the casters, Hazel, gets into debt with The Trader, a very powerful man with far reaching contacts must pay him back the value of her soul which is 100 souls.
Hazel goes on a quest to find and repay these souls.