Question

what is meant by 'Atomic mass unit is only number and it has no units'?

Answer 1

The atomic mass unit (abbreviated as amu or simply u) is a unit that is used to express the atomic masses of atoms and molecular masses of compounds. One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12.

The atomic masses of all elements have been found relative to an atom of carbon-12.