Using auto-generated numbers from custom variables as unique identifiers is not recommended. While these numbers are generated within a specified range, there"s a possibility that the same number could be assigned to multiple respondents. To provide each respondent with a unique identifier, you can display the Response ID on the Thank You page and prompt them to note it down, as each Response ID is unique to every collected response.

To include the Response ID on the Thank You page:​

1. Navigate to Finish Options in your survey settings.​

2. At the bottom of the Thank You page editor, locate the option to insert variables.​

3. Select Response ID from the available variables; it will be added to the Thank You page content.

Screenshot for your reference - https://app.screencast.com/Mo9SkaG77fJMP

Please refer to the helpfile for reference - https://www.questionpro.com/help/thank-you-page.html