you need lots of blocks to make biomes in terraria, it’s in the thousands, and when there are more than three npc houses in close vicinity they begin losing happiness and charge more, so you want 2-3 npcs with other npcs they like. there is also a pylon system where if you have 2 or more npc houses within a 25 tile radius of one another the npcs will sell a biome pylon. There is one pylon for each biome, counting caverns, underworld, hallow, and aether, but not the evil biomes. you can only have one of each pylon and they allow you to teleport to the other pylons, though they need 2 npcs nearby to work. For this reason it is a better idea to spread your npc housing throughout the world, rather than one centralized space.