You are double counting if you are adding up all of the players who are rated in any country or FIDE because some players are FIDE rated and rated in their home country. A few players are rated in multiple countries.
FWIW, Susan Polgar (quoting a source I don't recall) stated that there are 7.5 million players who are rated and 605 million people who know how to play chess (e.g., know how the pieces move).
Probably just trying to estimate it. I'm sure in the end he'd divide by some number he thinks is reasonable.
As for S Polgar, the source would be interesting.
USCF claims a membership of 85,000. Wikipedia, on the other hand, reports that USCF membership peaked at 85,000 in 2000, and has been steadily declining to about 77,000 in 2012/2013. http://en.wikipedia.org/wiki/United_States_Chess_Federation