  Many cold atom experiments rely on precise atom number detection, especially
in the context of quantum-enhanced metrology where effects at the single
particle level are important. Here, we investigate the limits of atom number
counting via resonant fluorescence detection for mesoscopic samples of trapped
atoms. We characterize the precision of these fluorescence measurements
beginning from the single-atom level up to more than one thousand. By
investigating the primary noise sources, we obtain single-atom resolution for
atom numbers as high as 1200. This capability is an essential prerequisite for
future experiments with highly entangled states of mesoscopic atomic ensembles.
