According to Official Guinness Records,
The largest gathering of people dressed as French maids is 715 people and was achieved by Migu Comic Co., Ltd. (China) in Xiamen, Fujian, China on 23 January 2016. The record was attempted by local comic book fanatics. In total 739 people took part in the event but 24 were discounted for not meeting the requirements.
For a complete list of 2016 records, please visit 2016 Guinness Records in China.