It looks like this parameters doesn’t work right now. I am using the last version of GP and WordPress (on a website in french)
Customizer > Blog > Blog Content.
I set “Extract”, and whaverer the number of lines I set (0, 1, 10), the full extract is displayed.
Instead, I had to use
Its the way WordPress works.
It checks to see if there is a Manual excerpt or a Read More.
If there is it displays it.
If not it takes the post content and passes it through a trim words function and displays that.
The reason is because both Manual and Read More allows the user to decide what to display for each post.
Overriding that with the word count would make both options unnecessary 🙂