Do NPCs in Terraria respawn?
If an NPC is killed, they will respawn after a delay, as long as a suitable vacant house is still available. If the NPC has an inventory or population requirement for their initial spawn (e.g. the 50 silver for the Merchant), that has to be met again for the respawn.
What time do NPCs spawn in Terraria?
If its daytime it will usually will have 1 NPC every 2 Minutes. If its night time the NPCs will not spawn. However it also affects if a bloodmoon has happened. If a Bloodmoon has occured the NPC will not spawn for the first 10 minutes of the day after.