The atomic number represents the number of protons in the nucleus of an atom. When an atom loses or gains electrons, it forms an ion. The atomic number of an ion remains the same as its parent atom.
Na+ is the sodium ion, which is formed when a sodium atom loses one electron. Sodium (Na) has an atomic number of 11 because it has 11 protons in its nucleus. When sodium loses one electron to become an ion, the number of protons remains the same, so the atomic number of Na+ is still 11.
He+ is the helium ion, which is formed when a helium atom loses both of its electrons. Helium (He) has an atomic number of 2 because it has 2 protons in its nucleus. When helium loses both electrons to become an ion, the number of protons remains the same, so the atomic number of He+ is still 2.