how many people come to your town in terraria

Answer 1

There's Merchant, Nurse, Demolitionist, Arms Dealer and Dryad. So five(Guide doesn't count).

Answer 2

 

First you need a house a (room that fills each requirement). Also each NPC needs a seperate house.

• Lightsource
• Walls and Background walls
• A door
• Tables and chairs

NPC Criteria

• Guide - You need a house

You get him at the start and he is only useful for tutorial help and crafting information.

• Merchant - 50 silver

Useful for Piggybank (portable bank slots), Mining helmet (See in the dark)

• Dryad - Must beat a boss (Cthulu, eater of worlds or Skeletron)

Useful for removing corruption and getting seeds.

• Nurse - Player has found a crystal heart. 

Useful for healing you for a price.

• Demolitionist - Must found an explosive in your inventory and Merchant needs to be inside a building and not dead.

Sells you explosives (bombs, grenades and dynamite)

• Arms dealer - Must have a gun either from shadow orbs or a chest. 

Sells you a gun, ammo and illegal gun parts (1 platinum)

• Clothier - You need to beat Skeletron (Oldman found near a dungeon) 

Sells you social items a helmet, mario outfit and black dye.

Extra NPCs you can aquire:

On a Multiplayer map you can get second and even third NPCs

• Second Nurse - Must have 3 players with atleast combined 400 hp in total.
• Second Merchant - 3 players with atleast combined 50 gold
• Third Merchant - 5 players with atleast 5 platinum in total

P.S. not sure what the secondary or the third merchants sell. (Presumably the same items as the first one)

[Reposting this again from my other answer to help abit]