unity - How do I handle loads of (prefab?) objects in Unity2D/3D?

I have been looking around for information regarding this process but I was unable to come across anything that would help me out. Basically, in a game where you may have many items or enemies or player objects, how would one go about creating these objects?

Am I supposed to be creating every single one of these objects as a prefab and then load them into the game when needed? Or is it possible to create one object that has references to the different variables needed to create the above objects and then load in from an external source all the necessary information? And then during run time, depending on what object I am looking at, the script will 'create' the object with the information from the external source.

Meaning, if I decide to have 50 different enemies in my game, do I need to make 50 prefabs, one for each enemy? Or just one prefab that loads in data pertaining to the 50 different enemies and then build upon that information?

Thank you!