Answer is: the mass number of a nitrogen atom is 13.
Atomic number (Z) is total number of protons and mass number (A) is total number of protons and neutrons in a nucleus:
p⁺(N) = 7; number of protons in nitrogen atom.
n°(N) = 6; number of neutrons in nitrogen atom.
A(N) = p⁺(N) + n°(N).
A(N) = 7 + 6.
A(N) = 13.