My first statistical information about polygamy was found from an online scholarly journal from EBSCO. This article showed how its data was collected which was by taking a sample of 2330 Protestants, Catholics, Polygamous Mormons, and other religions who responded to a series of questions regarding their sexuality, polygamy, religious orientation, religious doubt, and whether Mormons are considered Christian. The data was collected by Michael E. Nielsen out of the department of psychology at Georgia Southern University and Ryan T. Cragun out of the department of sociology at the University of Tampa.
Nielsen, M. E., & Cragun, R. T. (2010). Religious orientation, religious affiliation, and boundary maintenance: The case of polygamy. Mental Health, Religion & Culture, 13(7/8), 761-770. doi:10.1080/13674670902832805
The only government documents I could find did not have statistics on polygamy itself but I just found the laws against polygamy and the political votes on that were done for marriage laws to be passed, I will continue looking though!