We can know the valence of an element if we know in which is located.
For example if an element is in group IA it would have 1 valence electron,
if an element is located in group IIIA it would have 3 valence electrons, if an element is located in group VIIA, it would have 7 valence electron and so on.
Neon is located in group VIIIA, so it would have 8 valence electrons.