I'm working on a simulation game like SimTower where I'd like to keep track of thousands of items at once.

Other games like this, appear to have hundreds or thousands of rooms with elevators with what looks like hundreds or thousands of people. I'm concerned about achieving the same thing in my game.

How can I realistically keep track of each individual person/room/elevator in the game? I assume I can only render what's on the screen, but how can I afford to constantly simulate everything that is off the screen?