/**
 * {@inheritdoc}
 */
public function buildForm(array $form, FormStateInterface $form_state) {

  // get form config.
  $config = $this->config('MYMODULE.settings');

  // Set our count. Used in for loop below.
  if (!empty($form_state->getValue(['fieldset', 'field_count']))) {
    // get $form_state count value.
    $defaultCount = $form_state->getValue(['fieldset', 'field_count']);
  }
  else {
    // Get config count value.
    $defaultCount = $config->get('field_count');
  }

  $form['#tree'] = TRUE;

  // Fieldset item.
  $form['fieldset'] = [
    '#type' => 'fieldset',
    '#prefix' => '<div id="fieldset-wrapper">',
    '#suffix' => '</div>',
  ];

  // Field count item.
  $form['fieldset']['field_count'] = [
    '#type' => 'number',
    '#title' => 'Number',
    '#required' => TRUE,
    '#default_value' => $config->get('field_count') ?: 1,
  ];

  // Rebuild submit item.
  $form['fieldset']['rebuild'] = [
    '#type' => 'submit',
    '#value' => 'Apply number',
    '#submit' => ['::rebuildFormSubmit'],
    '#ajax' => [
      'callback' => '::fieldCallback',
      'wrapper' => 'fieldset-wrapper',
    ],
  ];

  // Loop through our count and create form items.
  for ($i = 0; $i < $defaultCount; $i++) {

    $form['fieldset']['field'][$i] = [
      '#type' => 'textfield',
      '#default_value' => $config->get('field')[$i] ?: '',
    ];

  }

  return parent::buildForm($form, $form_state);

}

/**
 * {@inheritdoc}
 */
public function fieldCallback(array &$form, FormStateInterface $form_state) {
  return $form['fieldset'];
}

/**
 * {@inheritdoc}
 */
public function rebuildFormSubmit(array &$form, FormStateInterface $form_state) {
  $form_state->setRebuild(TRUE);
}

It is important to use:

In the $form['fieldset']['rebuild'], along with:

Or you will get a Notice of undefined index from:

'#default_value' => $config->get('field')[$i] ?: '',

/**
 * {@inheritdoc}
 */
public function buildForm(array $form, FormStateInterface $form_state) {

  // get form config.
  $config = $this->config('MYMODULE.settings');

  // Set our count. Used in for loop below.
  if (!empty($form_state->getValue(['fieldset', 'field_count']))) {
    // get $form_state count value.
    $defaultCount = $form_state->getValue(['fieldset', 'field_count']);
  }
  else {
    // Get config count value.
    $defaultCount = $config->get('field_count');
  }

  $form['#tree'] = TRUE;

  // Fieldset item.
  $form['fieldset'] = [
    '#type' => 'fieldset',
    '#prefix' => '<div id="fieldset-wrapper">',
    '#suffix' => '</div>',
  ];

  // Field count item.
  $form['fieldset']['field_count'] = [
    '#type' => 'number',
    '#title' => 'Number',
    '#required' => TRUE,
    '#default_value' => $config->get('field_count') ?: 1,
  ];

  // Rebuild submit item.
  $form['fieldset']['rebuild'] = [
    '#type' => 'submit',
    '#value' => 'Apply number',
    '#submit' => ['::rebuildFormSubmit'],
    '#ajax' => [
      'callback' => '::fieldCallback',
      'wrapper' => 'fieldset-wrapper',
    ],
  ];

  // Loop through our count and create form items.
  for ($i = 0; $i < $defaultCount; $i++) {

    $form['fieldset']['field'][$i] = [
      '#type' => 'textfield',
      '#default_value' => isset($config->get('script')[$i]) ? $config->get('script')[$i] : '',
    ];

  }

  return parent::buildForm($form, $form_state);

}

/**
 * {@inheritdoc}
 */
public function fieldCallback(array &$form, FormStateInterface $form_state) {
  return $form['fieldset'];
}

/**
 * {@inheritdoc}
 */
public function rebuildFormSubmit(array &$form, FormStateInterface $form_state) {
  $form_state->setRebuild(TRUE);
}

This part in the $form['fieldset']['rebuild']:

along with:

Are the ajax options.