The following can be found on Sierra's Public Relations section:
Sierra Announces Development of Empire Earth: The Art of Conquest (May
"Released in November 2001, Empire Earth has become a worldwide hit,
with over one million copies sold."
Sierra lists Empire Earth as their #2 game on above page.
Even before the official release date of November 16, 2001, there was
high interest for what Charles Holtzclaw called "the next step in the
evolution of real-time strategy games":
Empire Earth Ships to Retail Stores (November 13, 2001)
"Gamers anxious for their holiday gaming fix have been clamoring for
Empire Earth, as witnessed by the more than 250,000 downloads of the
demonstration version released via the Internet on October 19th. In
addition, retail pre-orders for the game have been sky-high, thanks in
part to a special program that gave gamers a chance to participate in
a special beta-testing program."
At Amazon.com, Empire Earth is ranked at #45 for PC games:
(You can find other Amazon.com top sellers by clicking on the
left-hand categories of above page.)
Hope that answers it!
My original search strategy was not successful so I ended up browsing
the Sierra site "manually", but these were the terms used:
"Empire Earth" sales statistic
"game sales" worldwide 2001
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"computer charts" worldwide 2001