Limit REST API Posts Per Page

add_filter( 'rest_post_collection_params', 'wpfolks_limit_rest_posts_per_page' );

function wpfolks_limit_rest_posts_per_page( $params ) {
    if ( isset( $params['per_page']['maximum'] ) ) {
        $params['per_page']['maximum'] = 50;
    }

    return $params;
}