From pulse height to number of electrons
Assuming 1MOhm input resistance of the oscilloscope you can compute the current from the voltage and integrate it to get the charge. Divide by the charge of an electron and you get the number of electrons.

For the lower pulse that would give about -200nA * 15ns = -3e-15 C ~ 20000e.

Note that I'm not an expert w.r.t. photomultipliers, so I don't know whether this is of the right magnitude and whether there are some caveats of this measurement method.