How common is it for somebody to get traumatic nerve injury?

Highly variable. In order for it to happen one has to suffer trauma to a nerve or nerve plexus. This can happen from blunt trauma, lacerations, gun shot woulds etc.
Nerve damage. Traumatic nerve injuries are not that rare, mostly associated with motor vehicle accidents, workplace accidents and falls.

Related Questions

Which treatment works the best and fastest for traumatic nerve injury?

SCENAR. Good nutritional support (omega 3 supplement high in dha), and scenar treatment are two good starts. Scenar is a microcurrent biofeedback device that needs to be used by a certified and experienced practitioner, but can be particularly helpful. It's similar to a tens unit, but much more effective. Read more...