Legally? No permanent scarring, nor any maiming, nor deep bodily injury of any sort.
Wisely? You do what is neccessary, out of duty and love and not anger, and no more.
Spanking has different effects on different kids at different times of their lives. Some kids need their butt to remind their brain that they shouldn't do something. Some kids need the spanking to remember the words. Some kids are spurred to open and determined rebellion by the spanking. Some children are fully handled by a look of disappointment. Some kids need to be spanked to save their lives. Trying to make any further law than the no-maiming one, is ridiculous.