Question

How to find valence electrons using ionization energy?

Answer 1

Ionization Energy is the minimum energy required to remove the outermost electron from a gaseous neutral atom to form a cation.  

To find the number of valence electrons see where there is a significant change in the ionization energies.

For example the first ionization energy of an atom is 520 and the second ionization energy is 7297. Here a lot of difference is observed between first and second ionization energies. So from the data you can say that there is only 1 valence electron.