Answer here !

A mole is the amount of a substance that contains as many entities (atoms, molecules or other particles) as there are atoms in exactly 0.012 kg (or 12 g) of the carbon-12 isotope.

            From mass spectrometer we found that there are 6.023 × 10²³ atoms present in 12 g of C-12 isotope.

            The number of entities in 1 mol is so important that it is given a separate name and symbol known as Avogadro constant denoted by N_A.

            i.e. on the whole we can say that 1 mole is the collection of 6.02 × 10²³ entities. Here entities may represent atoms, ions, molecules or even pens, chair, paper etc also include in this but as this number (N_A) is very large therefore it is used only for very small things.