Depends on the size of the water, stock densities, methods employed and consideration for others.
Much as the use of four rods goes against the grain, as using three does to a lesser extent, I will do so when circumstances dictate. However, the water concerned is a thirty acre gravel pit that has never, to my knowledge, seen more than nine anglers at one time. It's a regular occurrence to be there all on your own. It's also home to very few fish, the chances of double hook ups are very, very slim. However, on a different water, as conditions dictate, or with certain methods, I've no probem with fishing with two or even one rod. On a massively overstocked commercial, I consider two rods to be excessive, especially when one of them is being used for float fishing, pole or rod.