In Diazonium Ion why does N contain 4 bonds?
One way to think about this structure, the benzenediazonium cation, is to consider the structure of benzonitrile - C6H5CN - just replace the positive nitrogen with a neutral carbon atom. This has the same basic electronic structure as the diazonium ion, but differs in that the carbon nucleus has one less positive charge than the nitrogen nucleus.

I don't think the concept of "coordinate bonds" is very helpful here. There are many different species with positively charged nitrogen, such as tetramethylammonium salts, that have four full covalent bonds to nitrogen.