From the Network Admin, go to Sites > All Sites. When click to edit a site you’ll be taken to a URL like
http://example.com/wp-admin/network/site-info.php?id=1
That number at the end is the site ID.
From the Network Admin, go to Sites > All Sites. When click to edit a site you’ll be taken to a URL like
http://example.com/wp-admin/network/site-info.php?id=1
That number at the end is the site ID.