Answer
4 electrons
Work Step by Step
Carbon atom is neutral when there are 6 electrons to counter positive charge of 6 protons of carbon nucleus. If one electron is removed from carbon atom now there are only 5 electrons present to counter positive charges of 6 proton.so one excess positive charge is designated as $C^{+}$. Similarly when two electrons are removed from carbon atom there are only 4 electrons present to counter positive charge of 6 protons, so 2 excess positive charges are designated as $C^{++}$.
