"It appears that sexually non-monogamous couples in the United States number in the millions. Estimates based on actually trying sexual non-monogamy are around 1.2 to 2.4 million. An estimate based solely on the agreement to allow satellite lovers is around 9.8 million. These millions include poly couples, swinging couples, gay male couples, and other sexually non-monogamous couples."
http://www.psychologytoday.com/blog/the-polyamorists-next-door/201405/how-many-polyamorists-are-there-in-the-us