Form elements as arrays #tree
This:

$form['subscribers_wrapper_container'][]["#tree"] = TRUE;
$form['subscribers_wrapper_container'][]['UCM_SUB'] = array(...


Can't be what you meant to do. You're creating two new elements there, one with only #tree in it.

Surely you want the tree on the outer element, e.g.

$form['subscribers_wrapper_container']['UCM_SUB']['#tree'] = TRUE;

for($i = 0; $i &lt; $form_state['values']['number_subscribers']; $i++){
  $form['subscribers_wrapper_container']['UCM_SUB'][] = array(
    '#type' =&gt; 'textfield',
    '#size' =&gt; 30,
    '#title' =&gt; t('Subscriber IP Address'),
    '#required' =&gt; TRUE,
  );
}