Dear Student,
Electronic configuration of nitrogen :1s2, 2s2, 2p3
Covalency : Covalency is the property by which a bond is formed due the sharing of electrons between two atoms.
Maximum covalency of nitrogen is 4 and it do not expand its covalency beyond 4 due to absence of d-orbital. For covalency beyond 4 in case of Nitrogen it require d orbital for the excitement of its electron to higher d-orbital. But as Nitrogen do not have d orbital so it has maximum valency 4.
Regards