Considering that they force it upon users and user cannot disable it, this sounds like a worthless metric.
I get an email every month telling me that my Copilot access has been renewed for another month. I'm probably being counted amongst those 20M users.
I could stand at the train station and yell "Cthulhu is our saviour" all day and later claim that the word of Cthulhu reached thousands of people today.