How do we calculate the number of protons, electrons and neutrons in an element?

The atomic number of an element is equal to the number of protons. It the element has a neutral charge then the number of protons equals the number of electrons. The number of neutrons equals the atomic mass minus the atomic number.

ex:

Sodium

Atomic number: 11

Atomic mass: 23

Protons: 11

Electrons: 11

Neutrons: 23 - 11 = 12