Requiring at least 2 check boxes be clicked using JavaScript

I am trying to build a form that requires at least two check boxes to be selected. I have tried everything that I could find without any sort of success.

function cb_require() {
  let chckbx = document.forms['form_name']['contact'].value;

  for (chckbx.length === 1) {
    if (i = 0; i < chckbx.length; i++) {
      alert('check 2');
      return false;
    }
  }
}

<form method="post" name="form_name" id="form_name" action="mailto:bb@yahoo.com" enctype="text/plain" onsubmit="return cb_require()">
  <label for="cont">Preferred Contact Method:</label>
  <input type="checkbox" id="Phone" name="contact" value="Phone">
  <label for="Phone">Phone</label>
  <input type="checkbox" id="E-mail" name="contact" value="E-email">
  <label for="E-mail">E-mail</label><br>
  <input type="checkbox" id="Mail" name="contact" value="Mail">
  <label for="Mail">Mail</label>
  <input type="checkbox" id="LinkedIn" name="contact" value="LinkedIn">
  <label for="LinkedIn">LinkedIn</label><br><br>
</form>

Thanks in advance for any help or advice!