Can vs. Should. Since coronary artery disease can affect all the major coronary arteries, the presence of significant blockage may occur in many vessels & branches. So the answer is dependent on the skill & courage of the operators, whether it be angioplasties & stents, or coronary artery bypass by arterial or venous grafts. Most patients get less than 6, but i've had colleagues that did up to 15 (in 1 patient).
As many as needed. There are three main arteries to the heart but many branches. There really is no theoretical limit to the number of bypasses. On average i would say 3-5 is typical. I have done more but not near as often. The most I have seen is 10!