Well two is a good optimization: it gives you a portable opposition point (with one arm you spend a lot of time pushing something against a wall or something like that -- try living with your arm in a cast) vs needing enough musculature to make the arms worth operating yet not make it heavy.

Plus there's a path dependency issue (the world is set up with chair backs, shirts, whatever assuming bilateral symmetry). But evolutionarily (reimagining the evolution of humans rather than adding an auxiliary arm) the path dependency is not meaningful.