Complex sentence: Avogadro's number was defined by Perrin as the number of molecules in one gram-molecule of oxygen and it is also the count of elementary entities as there are atoms in of the isotope carbon-12.
Split into simpler sentences: Avogadro's number was defined by Perrin as the number of molecules in one gram-molecule of oxygen. It is also same number of elementary entities as the number of atoms in of the isotope carbon-12.