Fast measurement methods are needed to determine the particle size distribution in environments with rapidly varying aerosol particle number concentrations. We used a MultiCPC system consisting of several CPCs with different cut-offs to study the timeframes of the changes in number concentrations at a semi-urban measurement site. With this system we could determine the number concentration of atmospheric aerosol in three size bins determined by the cut-offs of the instruments—the more instruments in the system, the more size bins could be obtained. The experiment demonstrated the ability of the system to measure the fast-changing particle number size distributions. The size distribution of sub-3 nm particles was simultaneously measured with an nCNC system, which demonstrated the need to include the nCNC in the MultiCPC system to measure particles starting from 1 nm, to see the bigger picture—during the day, even the number of 1.1–1.3 nm particles exceeded the number of all 23 nm–2.5 m particles.