Required

Required child

A required child must be filled in every row. The module does not add a blank row when the default values already fill the list, because a blank row would fail validation.

$form['names'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Names'),
  '#element' => [
    '#type' => 'textfield',
    '#title' => $this->t('Name'),
    '#required' => TRUE,
  ],
  '#default_value' => ['One', 'Two', 'Three'],
];

A list of three rows. Each input has the label Name and a star that marks it as required.

Required list

#required => TRUE on the element asks for at least one non-empty row. It also sets #min_items to 1.

$form['names'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Names'),
  '#required' => TRUE,
];

Submit the form with an empty list to see the error.

The form after a submit with an empty required list. The error message is at the top, and the empty row of the list is outlined in red.

Custom error

$form['names']['#required_error'] = $this->t('Add at least one name.');

Notes

A required child inside an element with #states is required only while the element is visible.
Related properties: #required, #min_items.