A study by the Pew Research Center, conducted in 2011 (Oct. 25-Nov. 16) surveyed the opinions of over 1000 self-identified Mormons, the first such survey by a group outside the LDS Church. Nearly all of the Mormons surveyed (99%) identified themselves as members of the LDS Church.
In the survey, there was a section covering questions of traditional morality, where one of the five questions was about polygamy. Responses...
Morally wrong 86%
Not a moral issue 11%
Depends 2%
Morally acceptable 2%
Not surprisingly, the number saying it was morally wrong depended on gender. Of Mormon women, 89% disapproved, compared to 83% of Mormon men.
How does that two percent translate into numbers of LDS Mormons believing that polygamy is acceptable? The 2010 census counted 234.5 million people in the USA over age 17. Of these, about 1.9% are Mormon, or 4.5 million. The two percent who accept polygamy would then number about 89,000.
The Mainstream Mormon Church
Known as the Church of Christ of Latter-Day Saints (LDS), it does not endorse polygamy, and excommunicates any members found to be practicing polygamy. Official statement