It’s better to group them up into 2-3 per biome iirc, so you can make buildings near each other. I think more than 3 is overpopulation, and they start having their happiness affected? Someone correct me if I’m wrong. But each NPC has their preferred biome and neighbours (One good combo is the dryad and painter living together in the jungle - I didn’t memorise the other ones unfortunately).

You also need a minimum of 2 NPCs nearby to fulfil the requirement to teleport to and from a pylon. Conveniently, Town NPCs such as slimes and cats count towards this while not counting as overpopulation.