Ajouter un formulaire et ajouter une valeur +1 au clic

 document.getElementById('fields').innerHTML += field;

 document.getElementById('fields').innerHTML =  document.getElementById('fields').innerHTML + field;

<div id="section2">

<div id="fields">

<div id="section2">
    <label>Episode 1 URL </label>
    <input type="text" name="url_ep1_s1" >
</div>

</div>
<div id="zone_bouton"> 
    <input type="button" onclick="addField()" value="Ajouter un épisode" >
</div>

function addField(){
  //container
  var zone_fields = document.getElementById('fields');
  
  //on compte le nombre de"fields" déjà existants
  var nb_fields = GetChildCount('fields');
  
  //Création des nouveaux éléments
  var newDiv = document.createElement("div");
      newDiv.id = "section"+(nb_fields+1);
  var newLabel =  document.createElement("label");
      newLabel.innerHTML = "Episode "+(nb_fields+1)+" URL";
  var newInput =  document.createElement("input");   
      newInput.type = "text";
      newInput.name = "url_ep"+(nb_fields+1)+"_s1" ;
      
  //Ajout des élements dans la zone_fields
  newDiv.insertBefore(newLabel,null);
  newDiv.insertBefore(newInput,null);
  zone_fields.insertBefore(newDiv,null);
  
}


function GetChildCount (container) {
  var container = document.getElementById(container);
  var childCount = 0;
  if ('childElementCount' in container) {
    childCount = container.childElementCount;
  }else {
    if (container.children) {
      childCount = container.children.length;
    } else {  // Firefox before version 3.5
      var child = container.firstChild;
      while (child) {
        if (child.nodeType == 1 /*Node.ELEMENT_NODE*/) {
          childCount++;
        }
       child = child.nextSibling;
      }
    }
  }
  return childCount;
}