The number is 40%, and it's 40% of code written by Copilot users. It's also just for Python:
> In files where it’s enabled, nearly 40% of code is being written by GitHub Copilot in popular coding languages, like Python—and we expect that to increase.
From what I remember they check in at a few intervals after the suggestion is made and use string matching to check how much of the Copilot-written code remains.
> In files where it’s enabled, nearly 40% of code is being written by GitHub Copilot in popular coding languages, like Python—and we expect that to increase.
https://github.blog/2022-06-21-github-copilot-is-generally-a...