How to have no more than 20 Entity references to another content type?
You should select unlimited as possible relations. Then, using a custom module, you can alter the node edit form to don't allow more than 20 items.

To do this, implement hook form alter:

function mymodule_form_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // do magic.
}


Inside you have to: 


Check the if the relations field has 20 item; if yes
delete the element widget that adds another one (so user don't get confused because it seems more items can be added).
Add a new validate handler to check related items are less than
21.