13C is an isotope of carbon having a mass number equal to 13. The mass number is the sum of the protons and neutrons in the nucleus. For a neutral carbon atom, the number of protons and electrons are equal. Thus, you don't need any additional information because you only have to know the number of protons to know the number of electrons. Since the atomic number of C is 6, then it has 6 electrons in its atom.
