As far as I'm aware it is not possible to change the maximum number of players without restarting the Minecraft daemon. What requirements do you have? Do you want it to react to the tickrate or the amount of players on, etc.?
Oh NPCs. I think that's deceptive and not going to make your server very popular but ok. http://dev.bukkit.org/server-mods/citizens/
There's Citizens but that one isn't really designed for deceiving people. If you just search dev.bukkit.org for "NPC" I'm sure you'll find plenty.