If you add one electron to an atom, the atomic number of the atom remains the same. The atomic number is the number of protons in the nucleus of an atom, and adding an electron does not change the number of protons.
However, adding an electron to an atom does affect the mass of the atom. The mass number of an atom is the sum of the protons and neutrons in its nucleus. While adding one electron does not directly affect the number of protons or neutrons, it does contribute to the overall mass of the atom, although the mass of an electron is much smaller compared to the mass of a proton or neutron.
In summary, when you add one electron to an atom, the atomic number remains the same, but the mass number increases slightly due to the addition of the electron's mass.