The atomic number for carbon 13 is 6.
Carbon-13 is one of the isotopes of carbon, which means it has the same number of protons as the most common form of carbon, carbon-12, but a different number of neutrons. All isotopes of an element share the same atomic number, which is defined by the number of protons in the nucleus. Since carbon has 6 protons, any isotope, including carbon-13, has an atomic number of 6. This is what defines carbon as an element, as the atomic number uniquely identifies it in the periodic table.