The covalence of nitrogen in N2O5 is 4.
When an element shares electrons with other atoms of the same or different elements to acquire a 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 the nitrogen atom to acquire a 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 pi-pi multiple bonds. Nitrogen shares it four pair of electrons with oxygen, therefore nitrogen covalency is four (4) in N2O5