Hello there!

Greetings!

One mole is the amount of substance containing as many elementary entities (atoms, molecules, ions, electrons, radicals, etc.) as there are atoms in 12 grams of carbon-12 (6.023 x 10^23). The mass of one mole of a substance equals its relative molecular mass expressed in grams.
So simply, the number of the particles present in one mole of any substance = 6.022x10 ^23 . This is called avogadros number or Avogadros constant.
For example, No. Of particles in 1 mole:1 mole of hydrogen atoms represents 6.022 10^23 hydrogen atoms.1 mole of hydrogen molecules represents 6.022 10^23 hydrogen molecules.1 mole of water molecules represents 6.022 10^23 water molecules.
The experiments with the help of which value of 1 mole was calculated is beyond understanding for common people , so I can't provide you that , it may be possible that you get the experiment for the same in your higher classes like during masters in chemistry.

Thankyou