Google Forms automatically assigns a unique, non-sequential and non-guessable identifier (id) to every form entry. This Unique Id contains a mix of alphabets and digits.
You can use this unique ID in subject line and message body of your email template with the help of dynamic form fields.
{{Response Id}}
The Unique Id is long and complex but if you wish to add a simple and sequential submission Id to your email template, you should use the response number instead of response id.
{{Response Number}}
The response number represents the sequential number of the current form response. For instance, if there are 10
form entries in your form, the Response Number for the next form submission will be 11.
A word of caution here. If there are 10 entries and the form owner manually deletes, say, the 8th response, the
next form entry will also be assigned the response number as 10. You should consider using
Response Id for truly unique identifiers.