Entity browser in inline entity form ignores cardinality

True. Should be pretty easy task. We need to read this setting from field storage and set it on entity browser form element.

Is this the right place to add this?

This belongs to entity_browser_entity_form_inline_entity_form_reference_form_alter().

Thanks for the feedback!

Another try, this time inside the form alter.

Looks good. Just one thing.

     '#type' => 'entity_browser',
     '#entity_browser' => $entity_browser_id,
+    '#cardinality' => $instance->getFieldStorageDefinition()->get('cardinality'),
   ];

What if there are already entities selected? EB will still allow full number of entities, which will end up more than allowed. We should check how many entities are already selected and substract.

True!
Attached another try. Not sure on how to fetch existing entities selected, though. This works, but feels kind of ugly

IEF stores selected entities in the form state. You are able to get them with:

$form_state->get([
    'inline_entity_form',
    $reference_form['#ief_id'],
    'entities',
  ]);

Much better! :)
Thanks!

Committed. Thanks!

This patch break the entity browser if used together with inline entity form.

The problem is, that you can only subtract the number of entities if the cardinality is NOT -1.

With this patch, the validation will fail, if you add 2 new items in the entity browser and later try to add an existing one with the entity browser.

I submitted a patch here:
https://www.drupal.org/node/2799299#comment-11618949