Yes, it is true that atomic mass is a dimensionless quantity and does not have a unit. The atomic mass of an atom is relative to the 1/12th mass of a carbon-12 (^12C) atom, which is defined as exactly 12 atomic mass units (AMU) or 12 Daltons (Da).
The atomic mass unit (AMU) or Dalton (Da) is a unit used to express atomic and molecular masses on a relative scale. It provides a convenient way to compare the masses of different atoms and molecules.
The value of 1 AMU or 1 Da is defined as exactly 1/12th the mass of a carbon-12 atom. This definition allows scientists to assign relative masses to other atoms and molecules based on their comparison to carbon-12. For example, an oxygen atom is about 16 times as massive as a carbon-12 atom, so its atomic mass is approximately 16 AMU or 16 Da.
In summary, while atomic mass itself is dimensionless, the atomic mass unit (AMU) or Dalton (Da) is a unit used to express the relative masses of atoms and molecules. It is based on the comparison to the mass of a carbon-12 atom.