In the "store" section of the create object action that give you the ability to remember the name of the object that was just created. you could immediately change its name using "set name" afterwards before making the next one.
If you combine an "int add" action as well as a "string build" action, you could take that name and increment its number by 1 over and over each time a new object is created.
so for example, your state that creates the objects would have the following actions
Create object (store name as gameobject)
get name (of game object just made)
Int add +1
Convert Int to string
Build string (game object name plus int)
set name (to the built string you just made)
Also When the newly a created prefab object is created it will automatically start executing its first "start" state if you have a FSM on it. So you could use that opportunity to rename it or whatever else as well.