Extremely uncommon.

The term "Gorilla rib" is used to describe a Lumbar rib. Most humans only have ribs bilaterally attached to their 12 thoracic vertebra but a number of variations exist with cervical ribs, lumbar ribs and even so called sacral ribs. Approximately 8% of humans have a extra rib attached to their lumbar vertebra which also occurs in some types of gorillas

It is an addition (13th) rib that is very rarely found in humans coming of of the 1st lumbar vertebral body. Apparently gorillas have 13 sets of ribs.

"Lumbar" refers to the region of the back or spine made up of the lumbar vertebrae. The 1st lumbar vertebra is just under the 12th thoracic vertebra. The easiest way to find this is to find the first vertebral body not connected to a rib. This is L1 (1st lumbar vertebra). There are 5 lumbar vertebrae (L1 through L5). The 5th lumbar vertebra sits on top of the sacrum (where your spine connects with your pelvis).

The thoracic vertebrae are found in the chest region of the body. Humans have twelve thoracic vertebrae which are located between the cervical and lumbar vertebrae.