How do you calculate the number of valence electrons?

Hint:The periodic table is defined as a table in which all the discovered chemical elements are organized by atomic numbers, from the element starting with the least atomic number i.e hydrogen, to the element with the maximum atomic number, oganesson. The atomic number of an element is known as the number of protons that are present in the nucleus of that particular element’s atom.

Complete answer:
A periodic table consists of 18 vertical groups and 7 horizontal rows.
ELECTRONIC CONFIGURATION OF ELEMENTS AND THE PERIODIC TABLE:
We know that an electron in an atom is characterised by a set of four quantum numbers (n, I, m and s) where the main energy level of the shell is denoted by the principal quantum number (n). The electrons get filled up respectively into different subshells which are s, p, d and f inside an atom. The way electrons are distributed into various subshells of an atom is actually the electronic configuration. The location of any particular element in the modern periodic table is characterized by the quantum numbers of the last orbital which gets filled. Here we will discuss the direct relation between the electronic configuration of the element and its location in the periodic table.
The number of valence electrons can be found out by using the group number. For example, group 1 elements have the valency as 1 like in case of Sodium. But for transition metals, we use proton numbers. Proton number is equal to the number of electrons as the atom is neutrally charged. Now, Chlorine has 7 valence electrons and that can be found out by looking at the electronic configuration that is $[Ne]3{{s}^{2}}3{{p}^{5}}$.

Note:
The value of n is indicated by the period or, for the outermost or valence shell. In other words, a successive period in the periodic table is associated with the filling of the next higher principal energy level like n = 1, n = 2, n = 3 refers to the filling of the first, second, third and higher principal energy levels. It can be readily observed that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled.