After receiving much demand for a auto incrementing number element, we have developed this feature and can be downloaded from this post.
The element’s value increases every time that the form is submitted, so every submission will have its own unique number. As a added feature you can also set the starting number from which the element starts incrementing.
To install this new element simply extract the attached file and add the autonumber.php file to the /components/com_philaform/elements directory.