Yes, in this sense it is countable. 'A light' is a normal expression.
In most rooms, there is 'a light' hanging from the ceiling. This refers to a single electrical installation, composed of a light fitting, an electric bulb ,and so on.
'A light' can also refer to a desk light or a bedside light in a room. 'A light' can also refer to a lamp, or a torch/flashlight.