Many vessels blocked. The two standard reasons for a coronary artery bypass operation are "triple-vessel" disease in which 3 or more of the primary or secondary vessels have significant blockages and "left-main" disease in which the main left coronary artery has a major blockage before it branches into the left anterior descending (lad) & circumflex arteries. There are other indications depending on the situation.
See below. There are 3 main ways to treat coronary artery disease. Medicine - stents - and surgery. Classically surgery is reserved for those patients when all 3 main arteries have blockages and the heart function is reduced. There are studies that show diabetics with 3 vessel disease do better with surgery. Also patients with blockage of the left main artery usually need surgery.