For a successful IT project manager, is it necessary to be good at everything or just needs to know how to assemble and manage a team with expertise?

Project Manager certainly need to know more than assembling and managing team of experts... Project managers need to understand the project he/she managing, need to understand different constraints (industry, stakeholders expectations, feasibility of project success/failure, benefit the project brings to the company, etc.). Project managers should be a good judgement maker, should be good at analyzing data and create projections, good at assumptions, a good thinker to come up with list of risks, stay on top of issues, a solid communicator...etc.. 

The more projects a person manages, the more maturity he gains as a PM..

I do agree with the answer given by Mr.: Salauddin  Mohammad