The experimental demonstration of a superconducting photon-number-resolving detector, based on the series connection of N superconducting nanowires, is presented. An integrated resistor is connected in parallel to each section of the device that provides in this way a single voltage-readout, proportional to the number of photons detected in distinct nanowires. As a proof of principle a four element detector has been fabricated from an NbN film on a GaAs substrate and fully characterized. Clearly separated output levels corresponding to the detection of n = 1-4 photons are observed achieving a single-photon system quantum efficiency of 2.6% at λ=1.3 μm. In order to demonstrate the potential scalability of the series-nanowire detector to a larger number of photons, we report our preliminary results in the characterization of detectors fabricated with 8 and 12 pixels. Clear evidence of n= 1-8 photon absorption in the 8-pixel detector has been achieved.