✓ Accepted answer
I'd advise first trying with humanoids, unless you're pretty decent with lua it's going to be a bit hard to do everything roblox does by default with Humanoids.
You can do a lot of simple things to make enemies seem more prevalent then they actually are, put in a lot of sharp corners if your idea supports that so they can't see too far ahead, and set about spawning them close to the player, and despawning them when they're out of a certain range.
You could also set about making a model that looks like the enemy that's just a set of anchored parts, and having the enemy automatically replace them when someone comes in range, on top of despawning and putting the model back when they leave range.