Min and empty items

The number of rows at first is the number of default values plus #empty_items. It is at least #min_items and at most #cardinality.

No rows at all

Set both to 0 and the table is replaced by a message. #no_items_message changes the message.

$form['notes'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Notes'),
  '#min_items' => 0,
  '#empty_items' => 0,
];

The message is No items entered. Please add items below.. To change it, set #no_items_message.

$form['notes']['#no_items_message'] = $this->t('No notes yet. Use the button below.');

At least some rows

#min_items keeps a number of rows on screen, even when there are no values. A row cannot be removed below it.

$form['notes'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Notes'),
  '#min_items' => 3,
];

Notes

• The default for #min_items is 0, or 1 when the element is #required.
• The default for #empty_items is 1.
• #item_label changes the word in the row button titles.
• Related properties: #min_items, #empty_items, #no_items_message, #item_label.