When we concept that one mole of a substance contains the same numbers of elementary entities as one mole of any other substance, we don’t actually need to know what that number is. Sometimes however we will need to work with the actual number of elementary entities in a mole of substance. This number is called Avogadro’s number.
N_A = 6.022137 × 10²³ molecules
The unit mol^–1 which we read as per mole signifies that a collection of NA molecular level entities is equivalent to one mole at the macroscopic level. For example a mole of carbon contains 6.02 × 10²³ atoms of C. A mole of oxygen gas contains 6.02 × 10²³ molecules of O₂.