Shoot the scene as many times as you have clones with the guy being the clone each time. (I would suggest using a portable green screen between the actor and the next chair.) Then, in AE, you'll just need to use the masks, Roto Brush, and/or keying to cut out each copy.
Do these clones need to be interactive? Do they raise their hands or talk to each other? Are they reacting to other characters or sounds in the scene?
Szalam's suggestion is a good place to start imagining how this would be shot. The details are more complicated than simply shooting the same guy in different chairs. You must make sure the camera doesn't move or, very important, shift focus or exposure. Do not refocus on each clone when you move the green screen setup. Some of the green screen keys you pull will be problems because the image will not be crisply in focus. Be careful not to move any of the props or you will end up wasting hours masking them out.
Carefully track which chairs have been shot and carefully slate each shot. It would help greatly if you practice a couple of shots and bring the footage into AE so you can understand your technical approach. And you will want to carefully label each layer in AE to correspond to the shot name or you'll get lost.
If your image does not need to be super photorealistic, you can assemble one row and clone it in AE and create the other three rows. But the perspective and lighting will be off slightly so don't expect this shortcut effect to totally sell the shot.
Listen to Bogesian.
As he says, the keys to success don't lie in After Effects as much as they do in careful planning, careful shooting techniques and TESTING. If you decide to just shoot this without a test, I have five whole American dollars that say you'll eventually wish you'd shot a test before you did it for real.
....and I just thought of a good cheat.
Instead of filling every chair in the room with this guy, why not just put him in selected spots spread across the room? Maybe 7 or 8 instead of 20? It cuts down on the degree if difficulty by quite a bit.