The number of valence electrons is the last number of that group -for example, all the elements in group 17 have 7 valence electrons, all the elements in group 5 (and group 15) have 5 valence electrons, etc.
From the number of valence electrons in an element you can infer how reactive it is, as generally those with more valence electrons are more reactive. So reactivity increases as you go across the periodic table from left to right, until you get the the noble gases. These have full valence shells so rarely react with anything (xenon is the most reactive out of these, because it is the biggest non-reactive noble gas. This means its valence electrons are further out from the nucleus, so there's less nuclear attraction, so it's easier for it to form bonds.)