It will call the ajax when customer click on the quantity button

add_quantity.js

jQuery(function( $ ) {
    $( "form.checkout" ).on( "click", "input.qty", function( e ) {
      var data = {
      action: 'update_order_review',
      security: wc_checkout_params.update_order_review_nonce,
      post_data: $( 'form.checkout' ).serialize()
    };

    jQuery.post( add_quantity.ajax_url, data, function( response )
    {
      $( 'body' ).trigger( 'update_checkout' );
    });
  });
});