Stents are wire mesh tubes that are used to prop open diseased blood vessels in the heart. These are usually placed during a procedure called percutaneous transluminal coronary angioplasty, whereby the vessel is blown up with a balloon and the stent is placed to keep it open. I am not aware of a "maximum" number, but location of the diseased artery, the overall health of the patient, and the frequency of previous procedures will decide if another stent is appropriate.