According to Official Guinness Records,
The most selfies (self-portraits) taken simultaneously is 4,991, achieved at an event organised by Younique Foundation (USA), in St. Louis, Missouri, USA, on 6 August 2016. The event was staged at America's Center to highlight Younique Foundation's new initiative: Defend the Innocence, organised to bring awareness of and help prevent child abuse.
For a complete list of 2016 records, please visit 2016 Guinness Records in United States.