Terraria

Most NPCs will appear once you meet certain criteria, which I've described below. You must house all of them to get the achievement. Note: In addition to meeting the requirements below, there must be an available, valid house in order for any NPC to spawn.

• Guide The first NPC you'll meet, spawns when you start a new world.
• Merchant Spawns when you acquire 50 Silver Coins total, in single or multiplayer.
• Nurse Spawns when you use your first Crystal Heart.
• Demolitionist Spawns when you acquire an explosive (Bombs, Grenades, and Dynamite) and have a Merchant NPC. Explosives are commonly found in chests and pots.
• Dryad Spawns when you have beaten a boss. King Slime doesn't count.
• Arms Dealer Spawns when you have acquired a gun or bullets. Bullets and guns are initially found in Shadow Orbs or Crimson Hearts.
• Clothier Spawns once you kill Skeletron.
• Mechanic Found in the Dungeon. You must talk to her to free her. She won't walk back to a house on her own, so you can either wait until she dies and respawns at your village (most likely to happen) or assign her to a house manually.
• Goblin Tinkerer Found in the Cavern layer after defeating a Goblin Invasion. You must talk to him to free him. He won't walk back to a house on his own, so you can either wait until he dies and respawns at your village or assign him to a house manually.
• Wizard Found in Hardmode in the Cavern layer. You must talk to him to free him. He won't walk back to a house on his own, so you can either wait until he dies and respawns at your village or assign him to a house manually.
• Dye Trader: Spawns once the player has any item used to make dye, or dye itself.
• Painter: Spawns once you have three other NPCs.
• Party Girl: Has a 2% chance to spawn each dawn once you have 8 or more NPCs.
• Witch Doctor: Spawns after defeating the Queen Bee.
• Pirate: Spawns after defeating the Pirate Invasion.
• Truffle: Spawns if there is an available house in a surface mushroom biome (you must manually create one) in Hardmode.
• Steampunker: Spawns after defeating one Hardmode Boss.
• Cyborg: Spawns after defeating Plantera.

Note: You don't need Santa Claus.

Most NPCs will appear once you meet certain criteria, which I've described below. You must house all of them to get the achievement. 

Note: In addition to meeting the requirements below, there must be an available, valid house in order for any NPC to spawn.
 

• Guide - The first NPC you'll meet, spawns when you start a new world.
• Merchant - Spawns when you acquire 50 Silver Coins total, in single or multiplayer.
• Nurse - Spawns when you use your first Crystal Heart.
• Demolitionist - Spawns when you acquire an explosive (Bombs, Grenades, and Dynamite) and have a Merchant NPC. Explosives are commonly found in chests and pots.
• Dryad - Spawns when you have beaten a boss. King Slime doesn't count.
• Arms Dealer - Spawns when you have acquired a gun or bullets. Bullets and guns are initially found in Shadow Orbs or Crimson Hearts.
• Clothier - Spawns once you kill Skeletron.
• Mechanic - Found in the Dungeon. You must talk to her to free her. She won't walk back to a house on her own, so you can either wait until she dies and respawns at your village (most likely to happen) or assign her to a house manually.
• Goblin Tinkerer - Found in the Cavern layer after defeating a Goblin Invasion. You must talk to him to free him. He won't walk back to a house on his own, so you can either wait until he dies and respawns at your village or assign him to a house manually.
• Wizard - Found in Hardmode in the Cavern layer. You must talk to him to free him. He won't walk back to a house on his own, so you can either wait until he dies and respawns at your village or assign him to a house manually.
• Dye Trader - Spawns once the player has any item used to make dye, or dye itself.
• Painter - Spawns once you have three other NPCs.
• Party Girl - Has a 2% chance to spawn each dawn once you have 8 or more NPCs.
• Witch Doctor - Spawns after defeating the Queen Bee.
• Pirate - Spawns after defeating the Pirate Invasion.
• Truffle - Spawns if there is an available house in a surface mushroom biome (you must manually create one) in Hardmode.
• Steampunker - Spawns after defeating one Hardmode Boss.
• Cyborg - Spawns after defeating Plantera.

Note: You don't need Santa Claus.