Avogadro’s number can be defined as number of atoms present in one gram atom or the number of molecules present in 1 gram molecule of substance. Avogadro’s number is the conversion factor among number of moles and number of individual particles. (if we want to know the number of atoms in an element we just multiply number of moles with Avogadro’s number similarly if we want to know number of moles in an element we just divide number of atoms by Avogadro’s number).
Mathematically,
Avogadro’s number = Number of atoms /(number of molecules in one gram atom / mole).
As Avogadro’s number is defined in numbers it is a dimensionless quantity.
SI unit of Avogadro’s number is mole-1.