I think it's based on the size of the soul and not the quantity of soul energy. Jason could have contracted all the elemental foxes if he had enough energy, so he'll probably be able to contract multiple flames. furthermore, they don't seem to take a lot of soul energy
There are, however, two disadvantages of Origin Flames, one is that the Origin Flame uses up a spot for soul beasts that not many humans are left with, once they find an Origin Flame.
Urban · HideousGrain
Author uploaded twice the same chapter
plot Armor will prevent it, no worries
In the end, they would figure out that he was not someone to laugh at after he brutally crushed them, and somehow deep within, Jason hoped that he would be matched against one of the few dozen students who had laughed at him.
God´s Eyes
Urban · HideousGrain