We present the modeling of a quantum regime for surface plasmon polaritons (SPPs) excited by an electron beam skimming parallel to the surface of a metallic structure. The theoretical approach resembles that used to describe the quantum Cherenkov radiation in which the quantized free electrons interact with a classical radiation field. In this paper, we rephrase the model in a more rigorous way, considering the detrimental effects of losses on coherent light. In the quantum regime of SPPs, each electron emits a single photon due to the transition between two successive momentum states. It is shown that the quantum nature of SPPs is realized in the low beam current limit where the radiation (gain in the field strength) operates in discrete frequency bands with a remarkably narrow linewidth. When losses are negligible, the photon emission occurs in periodic bursts along the interaction length. We show also that the Ohmic loss effects in the SPP process set an intrinsic limit on the coherent production of photons and also have severe detrimental effects on the radiation intensity. The findings of this study can describe recent experimental observations of the surface plasmonic near-field based on the photon-induced near-field electron microscopy.