How severe is the refractive error in a contact lens?

None. Contact lenses do not have refractive errors, but the eye may. Contact lenses come in different powers to compensate for the refractive error of the eye.
Matches your eye. Hopefully you have the right power in your lens. Soft lenses are made up to about 16 optical units and hard lenses can be made in any power. How high do you need?