diff --git src/wp-includes/sitemaps/class-wp-sitemaps-index.php src/wp-includes/sitemaps/class-wp-sitemaps-index.php
index 4ec1b07d92..4c93d18031 100644
--- src/wp-includes/sitemaps/class-wp-sitemaps-index.php
+++ src/wp-includes/sitemaps/class-wp-sitemaps-index.php
@@ -24,6 +24,15 @@ class WP_Sitemaps_Index {
 	 */
 	protected $registry;
 
+	/**
+	 * Maximum number of sitemaps to include in an index.
+	 *
+	 * @sincee 5.5.0
+	 *
+	 * @var int Maximum number of sitemaps.
+	 */
+	private $max_sitemaps = 50000;
+
 	/**
 	 * WP_Sitemaps_Index constructor.
 	 *
@@ -59,7 +68,7 @@ class WP_Sitemaps_Index {
 			array_push( $sitemaps, ...$sitemap_entries );
 		}
 
-		return $sitemaps;
+		return array_slice( $sitemaps, 0, $this->max_sitemaps, true );
 	}
 
 	/**
diff --git src/wp-includes/sitemaps/class-wp-sitemaps-registry.php src/wp-includes/sitemaps/class-wp-sitemaps-registry.php
index c232764924..c95f04c77f 100644
--- src/wp-includes/sitemaps/class-wp-sitemaps-registry.php
+++ src/wp-includes/sitemaps/class-wp-sitemaps-registry.php
@@ -24,15 +24,6 @@ class WP_Sitemaps_Registry {
 	 */
 	private $sitemaps = array();
 
-	/**
-	 * Maximum number of sitemaps to include in an index.
-	 *
-	 * @sincee 5.5.0
-	 *
-	 * @var int Maximum number of sitemaps.
-	 */
-	private $max_sitemaps = 50000;
-
 	/**
 	 * Adds a sitemap with route to the registry.
 	 *
@@ -69,19 +60,13 @@ class WP_Sitemaps_Registry {
 	}
 
 	/**
-	 * Lists all registered sitemaps.
+	 * Returns all registered sitemap providers.
 	 *
 	 * @since 5.5.0
 	 *
 	 * @return WP_Sitemaps_Provider[] Array of sitemap providers.
 	 */
 	public function get_sitemaps() {
-		$total_sitemaps = count( $this->sitemaps );
-
-		if ( $total_sitemaps > $this->max_sitemaps ) {
-			return array_slice( $this->sitemaps, 0, $this->max_sitemaps, true );
-		}
-
 		return $this->sitemaps;
 	}
 }
diff --git tests/phpunit/includes/bootstrap.php tests/phpunit/includes/bootstrap.php
index 817aea3bbe..8c88fc45e2 100644
--- tests/phpunit/includes/bootstrap.php
+++ tests/phpunit/includes/bootstrap.php
@@ -162,6 +162,7 @@ require __DIR__ . '/class-wp-rest-test-configurable-controller.php';
 require __DIR__ . '/class-wp-fake-block-type.php';
 require __DIR__ . '/class-wp-sitemaps-test-provider.php';
 require __DIR__ . '/class-wp-sitemaps-empty-test-provider.php';
+require __DIR__ . '/class-wp-sitemaps-large-test-provider.php';
 
 /**
  * A class to handle additional command line arguments passed to the script.
diff --git tests/phpunit/includes/class-wp-sitemaps-large-test-provider.php tests/phpunit/includes/class-wp-sitemaps-large-test-provider.php
new file mode 100644
index 0000000000..c2fbb11192
--- /dev/null
+++ tests/phpunit/includes/class-wp-sitemaps-large-test-provider.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * Class WP_Sitemaps_Large_Test_Provider.
+ *
+ * Provides test data for additional registered providers.
+ */
+class WP_Sitemaps_Large_Test_Provider extends WP_Sitemaps_Provider {
+	/**
+	 * WP_Sitemaps_Large_Test_Provider constructor.
+	 *
+	 * @param string $object_type Optional. Object type name to use. Default 'test'.
+	 */
+	public function __construct( $object_type = 'test' ) {
+		$this->object_type = $object_type;
+	}
+
+	/**
+	 * Gets a URL list for a sitemap.
+	 *
+	 * @param int    $page_num       Page of results.
+	 * @param string $object_subtype Optional. Object subtype name. Default empty.
+	 * @return array List of URLs for a sitemap.
+	 */
+	public function get_url_list( $page_num, $object_subtype = '' ) {
+		return array_fill(0, 1000000, array( 'loc' => home_url( '/' ) ) );
+	}
+
+	/**
+	 * Lists sitemap pages exposed by this provider.
+	 *
+	 * The returned data is used to populate the sitemap entries of the index.
+	 *
+	 * @return array[] Array of sitemap entries.
+	 */
+	public function get_sitemap_entries() {
+		return array_fill(0, 1000000, array( 'loc' => home_url( '/' ) ) );
+	}
+
+	/**
+	 * Query for determining the number of pages.
+	 *
+	 * @param string $object_subtype Optional. Object subtype. Default empty.
+	 * @return int Total number of pages.
+	 */
+	public function get_max_num_pages( $object_subtype = '' ) {
+		return 1000000;
+	}
+}
diff --git tests/phpunit/tests/sitemaps/sitemaps-index.php tests/phpunit/tests/sitemaps/sitemaps-index.php
index 5df604af8a..88b9294509 100644
--- tests/phpunit/tests/sitemaps/sitemaps-index.php
+++ tests/phpunit/tests/sitemaps/sitemaps-index.php
@@ -20,6 +20,16 @@ class Test_WP_Sitemaps_Index extends WP_UnitTestCase {
 		$this->assertCount( 24, $sitemap_index->get_sitemap_list() );
 	}
 
+	/**
+	 * @ticket 50666
+	 */
+	public function test_get_sitemap_list_limit() {
+		$registry = new WP_Sitemaps_Registry();
+		$registry->add_sitemap( 'foo', new WP_Sitemaps_Large_Test_Provider() );
+		$sitemap_index = new WP_Sitemaps_Index( $registry );
+		$this->assertCount( 50000, $sitemap_index->get_sitemap_list() );
+	}
+
 	public function test_get_sitemap_list_no_entries() {
 		$registry = new WP_Sitemaps_Registry();