The tl;dr of "Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons"

Avogadro’s number is the number of constituent particles contained in one mole of a substance. Its exact value is 6.02214076 × 10²³ mol⁻¹. The particles may be atoms, molecules, ions, electrons, or other specified entities. Avogadro’s number provides the bridge between the microscopic scale of individual particles and the macroscopic quantities measured in laboratories. It allows chemists to relate particle counts to measurable amounts such as mass and volume. Since 2019, its fixed value has also helped define the SI unit known as the mole.