The limits in mind depend on the kind of magic you practice and how far you're willing to go for power. Most of it involves channeling energy from one source, such as nature or a fellow witch, and converting it into something else. The limits there mostly depend on the power of the thing you're channeling and how much power you yourself can handle.

Some witches are willing to go farther to strengthen their own power, but there are still limits. A balance, if you will.