Multiple upload image using managed_file
In Drupal 8, add '#multiple' => TRUE

Example:

$form['images'] = array(
  '#type' =&gt; 'managed_file',
  '#upload_location' =&gt; 'private://images/',
  '#multiple' =&gt; TRUE,
  '#upload_validators' =&gt; array(
    'file_validate_extensions' =&gt; array('png gif jpg jpeg'),
    'file_validate_size' =&gt; array(25600000),
    'file_validate_image_resolution' =&gt; array('800x600', '400x300'),
  ),
);


More about form api: Introduction to Form API