NA61/SHINE is a large acceptance fixed target experiment at the CERN SPS which studies final hadronic states in interactions between various particles and nuclei. Preliminary results for mean negatively charged pion multiplicities $\langle \pi^- \rangle$ using the $h^-$ method are presented for central Ar+Sc collisions at 13, 19, 30, 40, 75 and 150A GeV/c beam momentum. The data were taken recently by the NA61/SHINE collaboration. Measured rapidity distributions $\frac{dn}{dy}$ were extrapolated to unmeasured regions to obtain total multiplicities $\langle \pi^{-} \rangle$. A new scheme to calculate the mean number of wounded nucleons $\langle W \rangle$ utilizing the EPOS MC model is described. Using also data from other experiments, a comparison of the ratio $\frac{\langle \pi \rangle}{\langle W \rangle}$ for different collisions and beam momenta is discussed.