add_filter('add_to_cart_fragments', 'chet_tips_woocommerce_header_add_to_cart_fragment');

function chet_tips_woocommerce_header_add_to_cart_fragment( $fragments ) {
  global $woocommerce;

  ob_start();

  ?>
  <span class="cart-count"><?php echo $woocommerce->cart->cart_contents_count; ?></span>
  <?php

  $fragments['span.cart-count'] = ob_get_clean();

  return $fragments;

}