Has anyone ever seen any information as to how many legal copies Windows 7 operating system (all version) are in use throughout the world today?
I can't give you an exact number but there was an article on Toms a few weeks ago which claimed that it has passed Windows XP in terms of deployment. I do not know if this includes all the pirated copies in Asia though.
My best guess would be based on the number of licenses Microsoft has sold. You'd have have to poke around their financial statements / press releases to figure that out. If the exact number isn't reported you could back into an estimate from their SEC filings. In the second quarter of 2012 they reported over 525 million sold, it's probably well over 600 million by now.
source: http://www.microsoft.com/investor/EarningsAndFinancials/Earnings/PressReleaseAndWebcast/FY12/Q2/default.aspx