Single particle inductively coupled plasma-mass spectrometry (spICP-MS) is an emerging technique that is capable of simultaneous measurement of the size and number concentration of metal-containing nanoparticles (NPs) at environmentally relevant levels. Although spICP-MS is widely applied to different fields, challenges remain in obtaining accurate and consistent particle number concentration (PNC) measurements. This paper presents, for the first time, a rigorous assessment of spICP-MS capabilities for measuring the PNC of gold NP (AuNP) suspensions of different sizes and coatings. The calibration of spICP-MS was accomplished with the National Institute of Standards and Technology (NIST) AuNP reference material (RM) 8013. The comparability of both spICP-MS direct and derived determination of PNC and reference PNC derived based on the mean particle size or the particle size distribution obtained by different reference sizing techniques was first assessed for NIST AuNP RM 8012, nominal diameter 30 nm. To enable a proper assessment of the accuracy of the spICP-MS results, a comprehensive estimation of the expanded uncertainty for PNC determination was carried out. Regardless of NP size or coating, a good agreement (90-110%) between spICP-MS direct determination of PNC and reported PNCs was obtained for all of the suspensions studied only when reliable in-house Au mass fractions and thorough mean particle size determinations were included in the calculation of the derived PNCs. The use of the particle size distribution over the mean size to derive PNCs resulted in larger differences for materials with a low contribution (<2%) of smaller NPs (30 nm), materials with a higher polydispersity (100 nm), or materials with two distinct subpopulations of particles (60 nm), regardless of NP coating.