Config entity form field with multiple/unlimited cardinality
(Content) fields have cardinality. There is no such concept for form elements.

However, entity_autcomplete does have a #tags flag, which you can see in the getInfo() method of the class: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21Element%21EntityAutocomplete.php/function/EntityAutocomplete%3A%3AgetInfo/8.

If you set that to TRUE, then it allows you to select a comma-separated list of references.

If you're looking for the "Add another" functionality of entity_autocomplete or any other form element, then you have to build it yourself.