Constants as globals aren't so bad. Personally I would still try to assign them to an appropriate owner class if there was one, but that's purely an organizational preference. That is, constants that have to do with physics would go in the class that manages the physics. Constants that have to do with the size of the map would go in the class that manages the map, etc.
An NPC list would likely be owned by either your map/world class, or whatever owns your map/world class. I typically do the latter.
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Map worldMap; // the world map
std::vector<NPC> npcList; // the npcs
You can then pass a Map pointer to each NPC that tells them what map they're interacting with (likely the pointer will just point to worldMap)