The Lewis structure depicts the number valence electrons surrounding an atom. These are the electrons present in the outer shells which can participate in bonding with another atom or remain as non-bonding (lone pair) electrons.
In the given example there are 7 dots or 7 electrons surrounding a Cl atom. This corresponds to 7 valence electrons with a configuration that can be given as: s²p⁵ and specifically, 2s²2p⁵
