Assuming your in Pre-Hardmode, you should just make a lot of houses for NPCs - more houses than the amount of NPCs you currently have.
Since you have 2 NPCs right now, maybe you should make 10 or 20 more houses.
I wouldn't particularly worry about getting a specific NPC, since you naturally get most of them by playing the game and progessing. For example, by exploring the world, you can find the Angler, Tavernkeep, Sylist, etc; by fighting bosses, you fulfill the requirements to get the the Witch Doctor, Goblin Tinkerer, etc; the list goes on. . .
You've probably inadvertently fulfilled the requirements to get a lot of NPCs already.