How rare is the condition intussusception among adults?

It is uncommon but: Not rare. I don't know exact incidence, but it happens from time to time. Important difference between children and adult cases is that the latter usually happens with a lead point, such as tumor or polyp. Therefore, surgery is almost always necessary. But these are general statements, so ask your doctor for specific advice.

Rare: In adult the cause is usually a "lead point" such as polyp or tumor either benign or malignant. Hence it warrants a totally different workup and treatment.