What does the number of electrons on the outer energy level of an atom tell you

Well, first off it tells you the number of valence electrons (because that's what those are valence electrons) it also tells you how that atom is going to bond with other atoms (whether it will tend to loose or gain electrons when in chemical bonds). It also can show how reactive an element is (if it has eight or in some cases two then it won't react because that outer shell is full) ... basically it tells you the chemical properties of that atom