There's no way to know. Microsoft doesn't seem to publich this, and they don't
even know with precision. Also, Excel is more often than not purchased as part
of Microsoft Office. There's no guarantee that all Office users are also Excel
users.
Unless Ziff-Davis or CNet keeps track of retail sales (and that'd be a poor
estimator), your best bet would be to see if there's any mention of this in
Microsoft's latest Annual Report or current SEC filings.
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