In ${\mathrm{NO}}_3^{-}$ ion, the number of bond pairs and lone pairs of electrons on nitrogen atom are

In N-atom, number of valence electrons = 5
Due to the presence of one negative charge, number of valence electrons = 5 + 1 = 6 one O-atom forms two (= bond) and two O-atoms shared with two electrons of N-atoms. Thus, 3O-atoms shared with 8 electrons of N-atom. 
$\therefore$ Number of bond pairs (or shared pairs) = 4
Number of lone pairs = 0