In a WordPress multisite setup, we use what is called blog ID to identify different child sites that are configured in the multisite setup. To find the blog ID of a child site, first log into your network admin configuration panel.
Go to Sites section on the left side, and choose the child site whose blog ID you want to see by clicking on its name.
After entering the details page, pay attention to the address bar. You will find it looks like this: http://domain.com/wp-admin/network/site-info.php?id=N, where N is your blog ID.