The number of experiments characterizing sub-3 nm aerosol particle dynamics has increased significantly over the recent years. In these experiments, it is essential to know/determine size resolved particle number concentrations accurately. Despite particle concentration measurement being relatively simple experiment, it can contain large uncertainties from various sources in the sub-3 nm size range. In this study we aim to identify and examine some of these sources. We simulated four different condensation particle counters (CPCs) (TSI 3777, ADI vWCPC, Airmodus A11 and an ideal CPC with d50 (lowest detection threshold) of 1.5 nm) and one differential mobility analyzer (DMA) (TSI nano DMA) and study the resulting uncertainties when using them to measure three different particle size distributions. First, we show that Poisson counting uncertainty root N represents the standard deviation, sigma, of the number of counted particles in all CPC and DMPS counting experiments. Second, the state-of-the-art DMA-CPC particle sizing system is examined with respect to counting statistics. Third, the performance of the instruments is assumed to be well-known, and instrumental non-idealities and the inversion routine are assessed. Fourth,+/- 0.5 nm offset is inserted to the CPC d50, and its effect on the measured particle concentration is examined. Our results highlight the importance of knowing the CPC d50 accurately to narrow down the particle concentration uncertainty. Furthermore, the results show that the current DMA-CPC measurements are subject to considerable counting uncertainty in low particle concentration environments. Based on the analysis we summarize suggestions for further research and instrumental development for more accurate sub-3 nm particle concentration measurements in the future.