Add prefabs to hierarchy without game running?

Here's a simple Editor Extension that lets you call any function on any given script. You don't have to attach this script to any gameobject, just leave it somewhere in your Assets folder. The specified script "YourScriptName" will have a button that, when clicked, calls the function "FunctionNameInYourScript". To call the function 100 times, just create a loop like you normally would. You could also forgo calling a function on YourScript, and directly instantiate from the extension code, or call a function that is defined in the editor extension script, or... Well, effectively, you can do anything in the Editor script that you want.

 using UnityEngine;
 #if UNITY_EDITOR
 using UnityEditor;
 #endif
 
 [CustomEditor(typeof(YourScriptName))]
 public class YourScriptNameExtension : Editor {
     
     #if UNITY_EDITOR
     override public void  OnInspectorGUI () {
         YourScriptName yourScriptName = (YourScriptName)target;
         if(GUILayout.Button("Name of the button")) {
             yourScriptName.FunctionNameInYourScript();
                     //If you want to directly instantiate a prefab, 
                     //just replace the above line with your instantiating code,
                     //and add "EditorUtility.SetDirty(instantiatedGameObject);" 
                     //after instantiating just to be sure, where "instantiatedGameObject"
                     //is the variable of your newly instantiated gameObject (NOT the prefab!)
         }
 
         if (GUI.changed) {
             EditorUtility.SetDirty(target);
         }
         DrawDefaultInspector();
     }
     #endif
 }