That's certainly an ambitious number, but it's certainly doable, mostly because PC makers will start installing Windows 8 on new PCs on October 26th. Despite the slowing sales growth of new PCs, there will still be a lot of them that will have Windows 8 on their hard drive.
Microsoft is playing the "my numbers are bigger than yours" with Apple. How about judging W8's success based on:
Sales of W8 Modern apps.
Increase of touch based device usage.
Decline of desktop usage.
Higher scores on user satisfaction surveys than previous version of Windows and their competition.
Profit from W8 licensing.
W8 is the beginning of Windows Next and if the modern UI, touch, etc. are a flop then IMO simply using the number of licenses sold as the measure of success of W8 won't help drive Microsoft and Windows forward.