How do you calculate the atomic number of an isotope?

Atomic number $Z$, is a given. The atomic numbers of isotopes are the same.
The number of protons, positively charged nuclear particles, determines the the identity of the element: $Z = 1 , H$; $Z = 2 , H e$; $Z = 3 , L i$; etc.
For hydrogen, for instance, ""^1H, the protium isotope (1 proton only in the nucleus), ""^2H, the deuterium isotope (a proton and a neutron in the nucleus) ""^3H, (a proton and 2 neutrons) the tritium isotope. These are all hydrogen atoms because they have the 1 proton in their nucleus, and are more or less the same chemically. Most elements have one or more isotopes. As $Z$ increases the range of isotopes becomes greater.