How do you determine the number of valence electrons?
By the family that the element is in.
You can simply tell how many valence electrons an element has by the family it is in. Sodium is in the first family, so it will have one single electron. When we get into the later half of the periodic table, i.e, into the double digits, we simply drop the 10. For example, our lovely carbon is in the 14th family. It has 4 valence electrons.