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.
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.
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.