gistfile1.js

  var num_obj = 1;

  document.querySelector('crowd-form').onsubmit = function(e) {
    const keypoints = document.querySelector('crowd-bounding-box').value.boundingBoxes || document.querySelector('crowd-bounding-box')._submittableValue.keypoints;
    const labels = keypoints.map(function(p) {
      return p.label;
    });

    if (labels.length != num_obj) {
      e.preventDefault();
      errorBox.innerHTML = '<crowd-alert type="error" dismissible>You must add exactly one bounding box on an object in the scene. If you have more than one drawn, please delete the additional ones and try to submit again.</crowd-alert>';
      errorBox.scrollIntoView();
      return;
    } 
  };