  We analyze the performance of photon-number-resolving (PNR) detectors and
introduce a figure of merit for the accuracy of such detectors. This figure of
merit is the (worst-case) probability that the photon-number-resolving detector
correctly predicts the input photon number. Simulations of various PNR
detectors based on multiplexed single-photon `click detectors' is performed. We
conclude that the required quantum efficiency is very high in order to achieve
even moderate (up to a handful) photon resolution, we derive the required
quantum efficiency as a function of the the maximal photon number one wants to
resolve, and we show that the number of click detectors required grows
quadratically with the maximal number of photons resolvable.
