How common is bowel obstruction aftrer c section?

Very, very rare. A true bowel obstruction is a mechanical blockage of the small intestine, usually due to scar tissue (adhesions) from prior surgery. An ileus is a functional obstruction of the intestine often due to inflammation within the abdominal cavity. While I have seen women with an ileus after c-sections, the only time I have seen a woman with an obstruction was associated with crohn's disease.
Why do you ask? Rare. This is not a question that comes up commonly in regard to c-section. Because the incision does not involve bowel at all (is between abdominal wall, bladder and gravid uterus) the risk for bowel to get caught up in it is very small, unless you have ovarian cancer, inflammatory bowel disease or peritonitis develop later. Cesarean section is not a risk factor for bowel obstruction!