The reason for only two eyes (and not one or four) is probably due to the limitations of embryological development and evolution.
The body is generally symmetrical, and has been for almost the entire evolution of multicellular animals. So left-right duplication is the default, making one eye development difficult.
The advantage of two eyes historically has been twice as much vision. In most animals (insects, fish, reptiles, birds, rodents), each eye handles vision on half of the body. Humans and monkeys are unusual in that both eyes face forward, have overlapping visual fields, and can rotate in their socket across a wide field of view. This makes 3D stereo vision possible. Owls also have front-facing eyes with stereo vision to help with hunting.