diff -u b/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
--- b/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -161,7 +161,7 @@
     switch ($cardinality) {
       case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
         $field_state = static::getWidgetState($parents, $field_name, $form_state);
-        $max = max(0, $field_state['items_count'] - 1);
+        $max = $field_state['items_count'];
         $is_multiple = TRUE;
         break;
 
@@ -179,7 +179,7 @@
     switch ($cardinality) {
       case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
         $field_state = static::getWidgetState($parents, $field_name, $form_state);
-        $max = $field_state['items_count'];
+        $max = (isset($form['#parents'][0]) && $form['#parents'][0] === 'default_value_input') ? max(0, $field_state['items_count'] - 1) : $field_state['items_count'];
         $is_multiple = TRUE;
         break;
 
only in patch2:
unchanged:
--- a/core/modules/field/tests/src/Functional/EntityReference/EntityReferenceFieldDefaultValueTest.php
+++ b/core/modules/field/tests/src/Functional/EntityReference/EntityReferenceFieldDefaultValueTest.php
@@ -147,11 +147,14 @@ public function testEntityReferenceDefaultConfigValue() {
     $field->save();
 
     // Set created node as default_value.
+    $this->drupalGet('admin/structure/types/manage/reference_content/fields/node.reference_content.' . $field_name);
     $field_edit = [
       'default_value_input[' . $field_name . '][0][target_id]' => $referenced_node_type->label() . ' (' . $referenced_node_type->id() . ')',
+    ];
+    $this->submitForm($field_edit, 'Add another item');
+    $field_edit = [
       'default_value_input[' . $field_name . '][1][target_id]' => $referenced_node_type2->label() . ' (' . $referenced_node_type2->id() . ')',
     ];
-    $this->drupalGet('admin/structure/types/manage/reference_content/fields/node.reference_content.' . $field_name);
     $this->submitForm($field_edit, 'Save settings');
 
     // Check that the field has a dependency on the default value.
only in patch2:
unchanged:
--- a/core/modules/system/tests/src/FunctionalJavascript/Form/RebuildTest.php
+++ b/core/modules/system/tests/src/FunctionalJavascript/Form/RebuildTest.php
@@ -52,8 +52,7 @@ protected function setUp(): void {
    * followed by a non-Ajax submission, which triggers a validation error.
    */
   public function testPreserveFormActionAfterAJAX() {
-    $page = $this->getSession()->getPage();
-    // Create a multi-valued field for 'page' nodes to use for Ajax testing.
+    // Create a multivalued field for 'page' nodes to use for Ajax testing.
     $field_name = 'field_ajax_test';
     FieldStorageConfig::create([
       'field_name' => $field_name,
@@ -96,7 +95,10 @@ public function testPreserveFormActionAfterAJAX() {
     // submission and verify it worked by ensuring the updated page has two text
     // field items in the field for which we just added an item.
     $this->drupalGet('node/add/page');
-    $page->find('css', '[value="Add another item"]')->click();
+    $field_edit = [
+      'title[0][value]' => $this->randomString(),
+    ];
+    $this->submitForm($field_edit, 'Add another item');
     $this->assertSession()->assertWaitOnAjaxRequest();
     $this->assertSession()->elementsCount('xpath', '//div[contains(@class, "field--name-field-ajax-test")]//input[@type="text"]', 2);