How can I get the "build number"

I’m new to Github Actions.
My question is:
What the number ‘5’ below calls in Github Actions ?
I know that in .travis.yml of Travis CI it calls $TRAVIS_BUILD_NUMBER .
Thank you!:smile:

I’m so sorry that I have not read the Context and expression syntax for GitHub Actions of Github Docs first.
The solution is ${{github.run_number}}.