Autocomplete on "Unlimited" text field?
You have to iterate your field's delta ; here, you only affect $form['field_company']['und']__[0]__, not  $form['field_company']['und']__[n]__

Stay in your hook_form_alter but iterate it each time your $form is rebuild :

    foreach($form['field_company']['und'] as $delta =&gt; $field) {
        if(is_array($field) &amp;&amp; is_numeric($delta)) {
            $form['field_company']['und'][$delta]['value']['#autocomplete_path'] = 'autocomplete/company';
        }
    }