The mass number of a nucleus (except Hydrogen) is:

The mass number of a nucleus (except Hydrogen) is: number of protons + number of neutrons.

A=Z+N

A=mass number=protons + neutrons.

Z=atomic number=number of protons.

N=number of neutrons.

In the case of Hydrogen it depends of isotope of Hydrogen.

the hydrogen has three isotopes,

protium : A=Z, because N=0

deuterium: A=Z+N; N=1

tritium: A=Z+N; N=2