[@UndeadSpartan]: In the case of something like a fireball, the ofuda itself would likely become the fireball. As for carry limitations it's basically however much you think your character reasonably can carry without it being noticed(provided they're going somewhere people might see them), honestly.

Familiars for onmyouji are shikigami, and shikigami are created by contracting formless spirits and giving them bodies. These formless spirits can have barely any actual consciousness of their own, as the body they are granted will give them a more complete sense of self.

Allowing a spirit to possess you would always be risky business, but I guess you could technically do it?