The covalence of nitrogen in N2O5 is 4.
When an element shares electrons with other atoms of the same or different elements to acquire stable electronic configuration, it is called covalency. If an atom shares 1 electron, its covalency is equal to 1. If it can share 2 electrons, its covalency is 2.
Nitrogen has 5 valence electrons. It needs 3 electrons to complete its octet. It can share three valence electrons with nitrogen atom to acquire stable electronic configuration to form N2.
Nitrogen combines with oxygen to form a number of oxides having various oxidation states. Nitrogen has a tendency to form pπ-pπ multiple bonds. Nitrogen shares it four pair of electrons with oxygen, therefore nitrogen covalency is four (4) in N2O5