Where can I find the GitHub ID in my account?

It can be easily retrieved using GitHub API:

https://api.github.com/users/your_github_user_name

where instead of your_github_user_name you must use the desired GitHub username.

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Example:
https://api.github.com/users/github

It can be easily retrieved using Github API.

Example: https://api.github.com/users/username

It can be easily retrieved using Github API. If you cannot use the API answer or from http://caius.github.io/github_id/ you can go to github --> settings --> emails , under the Primary email address you will find {id}+{user_name}@users.noreply.github.com. The format is simillar to this: [email protected], that 50826640 is the id.

From this answer

If you cannot use the API answer or from http://caius.github.io/github_id/ you can go to github --> settings --> emails, under the Primary email address you will find {id}+{user_name}@users.noreply.github.com.

This is at least true if you have Keep my email address private checked.
I have this text in the Keep my email address private explanation, and I'm guessing this will be there even if it is turned off.

Look for 'noreply' in the emails section.