Isotopes are atoms of the same element with the same number of protons and same number of electrons but with different number of neutrons. They differ in the number of neutrons that is why they also differ in mass numbers. Since they have the same number of protons, they have the same atomic number.
For instance, when the nucleus of the isotope with a 68.926 amu, there are 30 protons and 39 neutrons in the nucleus. The best example for this is Zn (zinc-69 isotope) and Ga (gallium-69 isotope) which has 68.926 amu and 68.925 amu respectively.