In general, as you move from right to left across a period, what happens to the number of valence electrons?
The number of valence electrons decrease in rows 1 ,2, and 3.
The atoms in group VIII A have eight valance electrons as you go to the right the group of VII A have seven valance electrons. This continues in the rows 1 2 and 3.
In the fourth row the d orbitals change things. The Group IV A has four valance electrons as you go to the right Group III A has three valance electrons. When you go one more space to the right Group II B has a total of 12 valance electrons 2 s electrons and 10 d electrons. Normally Zinc has a valance charge of +2 even though it has 12 valence electrons.
Then going to the right in the B families with d electrons each element to the right has fewer valance electrons.
So generally when moving from right to left the number of valance electrons decrease with a few exceptions.