Hi, I am quite new to unity community. Can anyone give me some advice regarding the spec for a server that can support 5000 concurrent user?
Note. that I am about to buy my own server for this project. 
It depends on your game.
My five cents,
Actually, no matter what kind of game you develop you do good by having as modern hardware as possible as they are more energy efficient. With time this will save both money and the environment.