An example of simple wpdb query script.

wpdb-get-max-id.php

<?php
/**
 * Returns max ID of the source wp_posts table.
 *
 * @return int
 */
public function get_source_max_article_id() {

	$in_source_post_statuses  = [ 'publish', 'draft', 'pending', 'private', 'trash', 'inherit' ];
	$post_status_placeholders = implode( ', ', array_fill( 0, count( $in_source_post_statuses ), '%s' ) );
	$prepare_values           = array_merge( [ $this->post_type ], $in_source_post_statuses );

	$source_db = self::$source_db_handle;
	$sql       = $source_db->prepare(
		"SELECT max(ID) FROM {$source_db->prefix}posts
		WHERE post_type=%s AND post_status IN ( {$post_status_placeholders} )",
		$prepare_values
	);

	return (int) $source_db->get_var( $sql );
}