How do I add conditional validation?
You can use the States API to make a field visually required, visible/hidden and so on.

One simple example from https://randyfay.com/states

  $form['tests_taken'] = array(
    '#type' =&gt; 'checkboxes',
    '#options' =&gt; drupal_map_assoc(array(t('SAT'), t('ACT'))),
    '#title' =&gt; t('What standardized tests did you take?'),
    '#states' =&gt; array(
      'visible' =&gt; array(   // action to take.
        ':input[name=student_type]' =&gt; array('value' =&gt; t('High School')),
      ),
    ),
  );


There are UI modules to configure this, like https://www.drupal.org/project/conditional_fields, but I think none is ported to D8 yet. So you have to form alter it in.

Also, keep in mind that this is UI only. You still need to implement corresponding server-side validation.