However many people it takes to change a light bulb, at least one of them will have to unscrew the old bulb from the socket and replace it with a new one. A hollow or cavity that you insert something into is a socket.
Many people also call electrical outlets (like the one you plug a toaster into) sockets. This makes sense because you are inserting a plug into it—some purists argue that it has to be called an outlet because power comes out of it, but it's a perfectly good way to use the word. There are also anatomical sockets in our bodies, like eye sockets into which eyeballs fit neatly, and ball-and-socket joints, like hips and shoulders.