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Since Tom opted out, I'll take a stab at your question - there are at least 4,000 'significant' art galleries in the US as of this time - here is link to many of them by state - http://art-collecting.com/galleries.htm .
Gleaning an estimate of the number of visitors to actual 'sale' galleries is somewhat difficult as most don't keep numbers of 'walk in guests'. By itself, the National Gallery of Art has thousands of visitors a year; whereas other galleries may do many sales by visiting clients homes, rather than clients visiting the gallery itself, or even online sales. I was unable to find any "published" numbers for gallery visitors in the US per year. You may be more successful if you specify a particular city or state you are most interested in, and work through individual galleries there.
Let me know if I can help further.