We studied the limits of optical reflectance to detect plasmonic nanoparticles (NPs) embedded in air and supported by a glass substrate, in an internal reflection configuration. We used a recently derived multiple-scattering model for the coherent reflectance of a disordered monolayer of particles supported on a flat surface to calculate the sensitivity of optical reflectance variations to the presence of plasmonic NPs. We considered gold and silver NPs with radii up to 50 nm and studied the sensitivity as a function of the angle of incidence, wavelength, particle size, and polarization of light. Using our own measurements of noise, we estimated the minimum detectable surface-coverage by the particles. The highest sensitivity is found around the critical angle between the glass substrate and air, for transverse-magnetic polarization, and at wavelengths of light near the plasmon resonance of the particles. We provide estimates of the minimum number of particles per unit area detectable from reflectivity variations and set the basis for single particle detection.