Some early statistics are shared on the update process 

http://www.wired.com/2015/07/14-million-people-already-using-windows-10/

http://blogs.windows.com/bloggingwindows/2015/07/30/windows-10-the-first-24-hours/

Windows 10 is barely two days old and it’s already huge. How huge? According to a blog post from Microsoft, 14 million computers are running Windows 10 merely 24 hours after its release. And this number will keep growing. Even with 14 million computers running, Microsoft has yet to meet the demand for Windows 10, and will keep rolling out upgrades in waves. So if you’ve reserved your copy of Windows 10, be patient and you’ll get your turn.