Atomic mass
Any mass defect due to nuclear binding energy is experimentally a small fraction (less than 1%) of the mass of equal number of free nucleons. When compared to the average mass per nucleon in carbon-12, which is moderately strongly-bound compared with other atoms, the mass defect of binding for most atoms is an even smaller fraction of a dalton (unified atomic mass unit, based on carbon-12). Since free protons and neutrons differ from each other in mass by a small fraction of a dalton (about 0.0014 u), rounding the relative isotopic mass, or the atomic mass of any given nuclide given in daltons to the nearest whole number always gives the nucleon count, or mass number. Additionally, the neutron count (neutron number) may then be derived by subtracting the number of protons (atomic number) from the mass number (nucleon count).
Milli mass unit
Since 1961 the unified atomic mass unit "u" has been defined as 1/12 the mass of C. Before that the atomic mass unit "amu" was defined as 1/16 the mass of O (physics) and as 1/16 the mass of O (chemistry). Thus the publication date in literature ought to be heeded when reading about the milli mass unit as its name does not reveal whether it refers to the old amu or the newer u.