How many diagrams is it best to use in an extended essay?

It's a common misconception that more diagrams are always better. In fact, a diagram that is ill-defined or poorly linked to the question will not add anything to your answer and it can demonstrate a lack of understanding, leading the examiner to penalise you in other cases where you haven't been clear about meaning. With this in mind, you should always put in at least one diagram, purely for the reason that every examiner will be looking for it as a 'box to tick', other than this only add extras if they demonstrate something yours doesn't, for example if the same economic stimulus in the question had occured in a market that was dominated by a monopoly, which could well be the case in the real world.