According to Official Guinness Records,
The most people blowing party blowers simultaneously was 6,961 and was achieved by Nana Mizuki fan club S.C. NANA NET (Japan) at the Tokyo Big Site, Tokyo, Japan, on 21 November 2009.
Nana Mizuki is a Japanese pop singer and voice actress. Her fan club members tried three record attempts at the concert. For this record attempt, 6,091 people out of 7014 audience successfully blew party blowers simultaneously for 10 seconds.
For a complete list of 2009 records, please visit 2009 Guinness Records in Japan.