Huh?
Minifigures are tied directly to buildings. Place a little red cottage somewhere, and after a little while the game chooses a semi-random minifigure to live there (each type of building has a list of minifigures that can live there), or multiple minifigures per building, depending on the type of building it is. Erasing a building will also despawn any minifigures tied to it. As far as I know, tunnels are only used for warping to different points along the edges of the map, sending mail to NPCs, and in multiplayer, sending mail to other players. Even then, if a minifigure gets on a train and the train is warped to another player's map (or if the tunnels are erased, or the depot is erased, etc), the minifigures will simply be teleported back to somewhere in the town.
So, basically, minifigures are tied to buildings, the only way to get more minifigures is to place more buildings, and the only way to remove minifigures is to erase the buildings they spawned from.
Minifigures also have jobs and stuff (office worker type people will sometimes go into skyscraper buildings, kids will go into a school building if it's accessible, etc), but I haven't messed with that much.