Most US clinics are members of ASRM. The age of the donor is the governing factor for DE transfers, and since most donors are under 35, they only recommend one or two (maximum). The Octomom disaster hit the news in February 2009, by November 2009 the ASRM came out with these specific guidelines:
http://www.asrm.org/uploadedFiles/ASRM_Content/News_and_Publications/Practice_Guidelines/Guidelines_and_Minimum_Standards/Guidelines_on_number_of_embryos%281%29.pdf