A maximum of 3 secret arts. Most recommendable is 1 to begin with.
Usage since it consumes a lot of magic power, I left it for you guys to decide. Make your secret arts meaningful and secret arts are basically finishers that are hax or pure damage.

EDIT: Aiden, Sable, and Cynthia with their current spells, list in the CS. 
Secret art making will be submitted here. Sides this is for ending the arc soon, finishing the boss with a secret art.