Every time you can independently assign the unique ID to each email/template. It can be the same number ID or different. And when you send this unique email to a lot amount of people it's whatever will be count like one message in the editor.
It means that you control the unique number of your messages on your own. So if you create the new email with another unique ID you will see number 2 in your account's menu.
Just notice that if you will assign one unique ID to few messages the auto-saving feature won't work and you can't save images correctly because in settings the folder path directly connect with email ID
Feel free to contact us if any other questions appear.