There’s no simple answer to this question. As long as your cat’s minimum requirements for protein are met, you have a lot of wiggle room to work with. Keep in mind, however, that the legumes frequently featured in grain-free cat foods have been linked to heart disease, though more study is needed in this area.
On the other side of the coin, a grain-inclusive diet featuring low-value grains like corn and wheat may be just as problematic.