Total number of charged particles as a function of $\sqrt{s_{\rm NN}}$ for the most central collisions at AGS (0-5% Au-Au) [25,26], SPS (0-5% Pb-Pb) [27,28], RHIC (0-5% and 0-6% Au-Au) [18,29,30] and LHC (0-5% Pb-Pb) [14]. The dotted, dashed, and full lines are extrapolations from fits to lower energy results [14], while the dash-dotted line is a fit over all energies, including $\sqrt{s_{\rm NN}}=5.02$ TeV.