Both the atomic mass and atomic number are the properties of an atom. The nucleus of the atom contain two particles: protons and neutrons, and it is these particles that determine the atomic mass and atomic number of the atom. The atomic number of an atom (or element) is the number of protons in the nucleus. Atomic mass, on the other hand, is the sum total of number of protons and neutrons in the nucleus of an atom. For example, sodium has an atomic number of 11 and an atomic mass of 23. This means that each atom of sodium contains 11 protons and 12 (= 23 - 11) neutrons. We can similarly determine the number of protons and neutrons in an atom, given its atomic number and atomic mass.
Hope this helps.