Slayer by Kiersten White

The long-awaited spin off of Buffy the Vampire Slayer, a New York Times bestseller.

In every generation, a Huntress is born.
Buffy was just a girl, a Chosen One. Only she was able to use her strength and her abilities to fight all kinds of creatures: vampires, demons, dark forces.
She was the only Huntress.
Until thousands of Potential Hunters are activated and the Chosen One ends up becoming one more among many Chosen Ones. The magic is about to disappear forever... and there is only one last Huntress left to summon. Only one young woman could save them all.
 
- Goodreads Summary