An element can have isotopes
Atoms of that element have an equal atom number, but a different mass number.
In nature, many elements are composed of some isotopes, and they occur is a fixed proportion.
The average atom mass of that element is then determined by the proportion of the various isotopes. The values normally can be found in tables.
Every atom has its own mass number: the sum of the nuclear particles.
The Clorine atom has 17 protons, and can have 18 neutrons. Or: the mass number of this isotope is 35.
But Chlorine can have 20 neutrons; then the Chlorine isotope has a mass number of 37.
The average atom mass of the element of Chlorine is based on the mass number of the different isotopes of the element.
Easy to believe that the average is 36, but that is wrong thinking.
In nature we find about three times mor 35Cl compared to 37Cl.
Finally you can calculate that the average atom mass of Chlorine is about 35,5.