Four or five. According to the query of 39 Health Network, each lumbar vein is composed of two branches, the back and the abdomen, and four or five lumbar veins flow into the renal vein. The dorsal branch receives venous blood from the waist, muscles and skin and communicates with the posterior vertebral venous plexus, the ventral branch receives venous blood from the abdominal wall, and the lumbar venous trunk communicates with the anterior vertebral venous plexus.