Question

does atomic mass equal number of protons plus the number of neutrons?

Answer 1

The atomic mass is the total mass of protons, neutrons and electrons in a single atom.
Read some more on the following link, there are things like 'Relative Atomic mass' and 'Atomic Weight' etc explained...
http://en.wikipedia.org/wiki/Atomic_mass

Answer 2

As electrons barely have any mass when compared to protons and neutrons, you could effectively just state that atomic mass is equal to # protons and # of neutrons.