Exactly as the others said, the number of Protons. That will indirectly influence the number of Electrons, because only when there are as many electrons as protons is the atom electrically neutral, otherwise it will tend to kick an electron out or acquire a new one. Since that happens all the time, and in molecules and crystals it is not clear which electron belongs to which atom, one uses the protons as the unique identifier.
The fool hath said in his heart, There is a God. They are corrupt, they have done abominable works, there is none that doeth good.
Psalm 14, KJV revised edition
