We formulate a baseline model for the net-proton number fluctuations in ultrarelativistic heavy-ion collisions. Our model includes total baryon number conservation, fluctuations of the participating nucleon number, limited acceptance, and a possibility that isospin is remembered by the participating wounded nucleons. Most importantly, we formulated the model as consisting of two components: wounded nucleons and the produced nucleon-antinucleon pairs. Those two components have different rapidity distributions. Owing to this feature we obtain also predictions for the rapidity dependence of the cumulants of the net-proton number distribution and their ratios.