Drupal Webform Select 2 options instead of unlimited
You can do client-side checking for the number of items selected, and if greater than 2, you can add your desired behavior, such as clearing the selections and/or alerting the user.

Example with jQuery:

var selectID = '#your-select-ID'; // change this to your selector
$(selectID).change(function () {
  if ($(selectID + " option:selected").length &gt; 2) {
      $(selectID).val([]); // clear the selections
      // do other stuff if you want, like alert('Can only select 2 options');
  }
});