What is a valence electron and how many does each element want?

Valance electrons are the extra electrons on the outermost orbital, that hasn't been filled yet. For example, Lithium would have one valence electron, on the 2s orbital, because that is an extra electron floating around that 2s orbital, that can be shared with others, since each element's goal is to have no valence electrons. If an orbital is filled, it therefore has no valence electrons, because valence electrons are only extra electrons, if that makes any sense.

There are many groups of the periodic table. These groups categorize electrons by the amount of valence electrons they have. The group on the far left, the alkali metals, all have one valence electrons. The alkali metals include Lithium, Sodium, and other elements, so therefore give away that one electron easily, because if they give it away they won't have any valence electrons. (This is why you often see salt, sodium chloride, because sodium gives away its one valence electron to chloride, who needs one electron to have no valence electrons.) One other notable group is the noble gases, on the far right of the periodic table. These elements have no valence electrons, so therefore won't bond easily with other elements.

Hope this helped!