The mass of Carbon-12 atom is 12.0000000 a.m.u. For relative isotopic mass, e.g. Mg-24 atom is 24 compare with the c-12 as standard. However, why is it unitless? Shouldn't it be 24 a.m.u.?
Relative atomic mass is a dimensionless quantity. It is relative mass of a specie with respect to the mass of carbon-12. So the units of masses get cancelled and hence we have a 'unitless' quantity. Just like refractive index doesn't have a unit. Physical quantity that are a result of ratio don't have units as they get cancelled.