How common is it for somebody to get posterior inferior cerebellar artery syndrome?

Depends: In the general population, it is rare. However, among people with strokes, it is common enough to have an eponym (wallenberg syndrome).

Not common: This an uncommon type of circulation problem in the lower, back part of the brain called cerebellum. A blood vessel tear, a clot traveling from the heart to the brain or local arteriosclerosis can cause this stroke. It is not as common as strokes affected to front part of the brain.