Jquery - Validating only one form out of multiple forms

If any div with form is dynamically added then create one function that apply validate on form based on Id and call function when div or form added dynamically.

    function applyValidate(id){
       $(id).validate({
         ignore: '',
         onsubmit : false,
         rules: {
           comment: {
             required: true,
             minlength: 10
           }
         }
      });
   } 

If all forms are loaded in page at a time and You want to bind validate on each form then you can use below code.

$(function(){
  $(document).on('submit', '.comment_form', function(e){
    form_id = $(this).id
    e.preventDefault();    
    if($(this).valid()){
      console.log('success')
    }
  });
	
  /* If all forms are not dynamically added and rendered at once in page then you have to apply validate on each form otherwise add applyValidate(id) function outside of $(function(){}); and call applyValidate(id) function when form is dynamically added*/
  
  $('.comment_form').each(function() {  
    // attach to all form elements on page
    $(this).validate({
      ignore: '',
      onsubmit : false,
      rules: {
        comment: {
          required: true,
          minlength: 10
        }
      }
    });
  });
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.15.1/jquery.validate.js"></script>


<div>
  <p> Some Answer </p>
  <div class="comments"></div>
  <form id="form_62" class="comment_form" method="POST" action="URL"> <!--This id is variable-->
    <textarea rows="3" name="comment"></textarea>
	<button type="submit">Comment </button>
  </form>
</div>

<div>
  <p> Some Answer </p>
  <div class="comments"></div>
  <form id="form_63" class="comment_form" method="POST" action="URL"> <!--This id is variable-->
    <textarea rows="3" name="comment"></textarea>
	<button type="submit">Comment </button>
  </form>
</div>

<div>
  <p> Some Answer </p>
  <div class="comments"></div>
  <form id="form_64" class="comment_form" method="POST" action="URL"> <!--This id is variable-->
    <textarea rows="3" name="comment"></textarea>
	<button type="submit">Comment </button>
  </form>
</div>

Problems with your code called out in my comments:

$(function(){

    // can NOT use submit handler since the validate plugin is already capturing this event
    $(document).on('submit', '.comment_form', function(e){ 

        ....
        // can NOT use 'valid()' before 'validate()' has initialized
        if($(this).valid()){ 
            ....
        }
    })

    // $(this) is meaningless here since it's not inside a pending jQuery method being invoked
    $(this).validate({
        ....
    });
})

You would not call .validate() when you submit the form. The .validate() method is only used to initialize the plugin on your form, not call validation of the form. So normally .validate() is called when the page is loaded.

Since you have forms that do not yet exist when the page is loaded, you would only call .validate() as soon as the dynamic form is created, not submitted. In other words, invoke the .validate() method as the very last step within whatever function creates this form, and since your submit button is a type="submit" the validate plugin will capture it automatically and you will not need a submit or click handler to get validation.

$(function(){

    function create_dynamic_form() {

        // code to dynamically create this form HERE


        // initialize validation on this new form ->

        $('#this-new-form-id').validate({
            ....
        });

    });

});

My example could be more efficient if you had shown us the actual code that dynamically creates the new form.