Questions about creating a server?!

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Hey guys,
So I have a game idea, but want to know “how difficult would it be to create a server that holds 1,000-5,000 people?” I know thats a pretty big server for someone to be even asking. And along with that “what knowledge do you need (or just recommended) to do this?”

In addition to that is there any 3rd party company or people, that would be able to provide a big server like that for a game?

Any thing will help, I get some of this may be out of reach for this engine, but i’m looking for ideas and/or options.