There are 3 shells total in the atom of Chlorine. The first shell has 2 electrons because the first shell of any atom can only hold up to a maximum of 2 electrons. The second shell holds 8 electrons because the second shell of any atom can only hold 8 electrons maximum. The third shell holds 7 electrons in the Chlorine atom, but the third shell of any atom can hold up to 18 electrons as a maximum.