How do you determine the atomic number?
From the periodic table, the number of protons or number of electrons when there is no charge on the atom.
The atomic number is always listed on the periodic table ranging from the smallest and increasing along the period and down the groups. The atomic number is always the smaller number when two appear for the same element, the other one is the atomic mass which includes the relative number of neutrons.
The atomic number is determined by the number of protons present in the nucleus. This is the more reliable way as the proton number does not change for the specific element's isotopes.
The electron number when there is neutral charge on the element can also be used but as it is not unusual for atoms to lose or gain electrons and become ions this is not as commonly used.
There isn't really any way for us to really calculate the atomic number if that's what you're worried about being tested on. The best way is to use the periodic table, or even memorize it since the common ones will be used a lot.