I am fairly new to this game and haven't played old versions yet where the NPC's were in the game, so I am not really sure how the NPC's would behave and if they would just stay in their houses idle until you come or roam around like you, scavenging stuff. In last case, they would probably die a lot and thus there should either be a lot survivors at the beginning or code the game this way, that always when a survivor dies, another one is randomly generated into the game somewhere. It's also realistic as survivors from other towns might be on a trip somewhere else and make stop at this town, which would explain new survivors.