What is the maximum number of elements allowed in a form?
I think, the issue is related to Server configuration, not with Drupal.

There is max_input_vars variable in php.ini which define the maximum number of variable form elements that can be submitted to a single page.  


  The “max_input_vars” setting in the php.ini file is a relatively new
  option starting last year from PHP 5.3.9. It allows you to control the
  maximum number of form elements on a page. Some webpages have a large
  amount of forms/fields for administrative or page configuration
  purposes. In those cases, we need to increase the number of this value
  in order to properly view those pages.


There is also post_max_size variable in php.ini.


  post_max_size sets max size of post data allowed. This setting also affects file
  upload. To upload large files, this value must be larger than
  upload_max_filesize. If memory limit is enabled by your configure
  script, memory_limit also affects file uploading. Generally speaking,
  memory_limit should be larger than post_max_size. If the size of post data 
  is greater than post_max_size, the $_POST and $_FILES superglobals are empty.


You can check the this for more details.