How to validate a Cloned form fields by javascript?

// Cloning Form
  var id_count = 1;
  $('.add').on('click', function() {
    var source = $('.form-holder:first'), clone = source.clone();
    clone.find(':input').val("").attr('name', function(i, val) {
      return val + '-'+id_count;
    });

    clone.appendTo('.form-holder-append');
      id_count++;

  });

  // Removing Form Field
  $('body').on('click', '.remove', function() {
    var closest = $(this).closest('.form-holder').remove();
  });

.remove
{
display:none;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="form-folder">

 <input type="file" name="variant_images" id="upload_file" required multiple>
  
 
  
  <button class="remove">remove -</button>
</div>



  <div class="form-holder-append"></div><br>
  <button class="add">Add +</butoon>
  

// Cloning Form
  var id_count = 1;
  $('.add').on('click', function() {
    var source = $('.form-holder:first'), clone = source.clone();
    clone.find(':input').val("").attr('name', function(i, val) {
      return val + '-'+id_count;
    });

    clone.appendTo('.form-holder-append');
      id_count++;

  });

  // Removing Form Field
  $('body').on('click', '.remove', function() {
    var closest = $(this).closest('.form-holder').remove();
  });





//Image upload restriction
$(document).ready(function(){
  $('#upload_file').change(function(){
   //get the input and the file list
    var images = document.getElementById('upload_file');
      if(images.files.length<4){
          alert ("Please Upload minimum 4 images.");
          $("#upload_file").val('');

      }
   });
});

.remove
{
display:none;
}
.form-holder-append .remove {
  display: block;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="form-holder">

  <input type="file" name="variant_images" id="upload_file" required multiple>
 <button class="remove">remove - </button>

</div>
<div class="form-holder-append"></div><br>
<button class="add">add + </button>