Why do you think the mass number includes only the number of protons and neutrons?

The mass of electrons are so small that it is pretty much insignificant.

Explanation:

mass of proton = 1.673*10^-27

mass of neutron = 1.675*10^-27

mass of electron = 9.109*10^-31

the electron's mass is about 0.0005 times the mass of a neutron