i always had the impression that the more perfect your town is and the more your museum was filled up, the more people would move in your town. i think i remember nook or lyle telling me something like that in one of the older versions of the game. i could be hallucinating, though.
You can have up to 10, no more. If you want to get someone out, hit them with a net, complain about them, make holes around there homes, set pitfalls, ignore them if they want to talk to you, and if they plan to move, don't stop them. Do not write to them. Do not talk to them.
You can't have any more than ten neighbors from what I've heard. You can Time Travel a ton into the future, however, you will have no control as to who moves in or out.