How is the atomic number of an element determined?
By the number of protons found in one atom of an element.
The atomic number tells us the number of proton atoms in a single atom of an element. It is unique to each element.
Do not be confused with isotopes, where an has the same number of protons (as its other isotopes), but a different number of neutrons.
In this example, Hydrogen's atomic number remains the same - at 1. The only difference are the number of neutrons.
Hope this helps :)