I have the spare bodies in my inventory but the NPCs still don't spawn, do i need to do anything else to make them move in?

I haven't made NPCs to go with some spare bodies, the ones I have will say "Allows ___ to move in" The list of completed NPC are in the description. If this does not help, let me know an I can try and resolve the issue

how to make them spawn?

You just need to keep them in your inventory, the ones I have sprites for will say Allows ___ to move in. Just make sure you have available housing and the correct spare body