A nitrogen atom forms three covalent bonds. The number of valence electrons an atom possesses determines how many covalent bonds it can form. Since nitrogen has five valence electrons and bonds, it uses three of its five valance electrons for bonding.
Even though nitrogen has five valence electrons, it is unable to form five covalent bonds. Nitrogen and other atoms make bonds so that they have a total of eight electrons, giving them the same electron configuration as a noble gas. This makes the atom much more stable than it would otherwise be. Since nitrogen only needs three more electrons to reach the eight it needs, it only forms three bonds.