Why does counting is not consistent in Parallel.For?

Question

  var items = Enumerable.Range(0, 20);
  int cnt = 0;

            Parallel.For(0, items.Count(), (int i) => {
                WorkOnItem(items.ToArray()[i]);
                cnt++;
            });

static void WorkOnItem(object item)
        {
            Console.WriteLine("Working on item:" + item);
            Thread.Sleep(10);
        }        Does it mean the thread failed?