Theoretically, how many players can a single server hold?

So I've been thinking a lot about the future of my server and I ran into an interesting topic. How many players can a single server (this means no bungee or lily pad) hold? As a factions server owner, I do not want to turn my server into a generic hub, but I still want to grow and have the hardware to support as many players as possible.

So what does it come down to, the processor right? What is the best server processor on the market (that is reasonably priced), the i7-4770k? How many players...

Theoretically, how many players can a single server hold?