What is the total number of protons and neutrons in an atom called?

This is called the Mass Number of the isotope of the atom.

Isotopes are different forms of the same atom. They have the same number of protons but different number of neutrons.

Carbon-12 has mass number 12 as it has 6 protons and 6 neutrons.

Carbon-14 has mass number 14 as its has 6 protons and 8 neutrons.