I believe there are 18 valves in the engine as there are 6 cylinders. I expect each cylinder to have at least one spark plug.

Mercedes are slow to change to 4-valves per cylinder engines and I am only aware of Alpha Romeo to have two spark plugs per cylinder on their two valve per cylinder engines.

The fact you have asked the question make me wonder if a 'spark plug' could be used for other duties in the XF. Can't think of anything myself, and I don't carry any spares with me.