What does the mass number indicates in an element?

Answer 1

The atomic number (or mass number) is the proton number (the number of protons in the nucleus) added to the number of neutrons in the nucleus.

The Atomic Mass is, for stable isotopes, roughly twice that of the proton number although the higher the proton number is the more the number of neutrons increases but this is most apparent in the very heavy elements.

It is mainly useful in the use of decay equations as the neutrons lead to the stability of the atom so, when the atomic mass is above or below normal the atom can become unstable and is likely to undergo radioactive decay.

Answer 2

The atomic mass number is the total (sum) of number of protons and number of neutrons of a specific atom (it is isotopically specific, not specific for an element)