According to Official Guinness Records,
The largest gathering of people dressed as Wally/Waldo is 4,626 and was achieved by Huis Ten Bosch Co., Ltd. (Japan) at Huis Ten Bosch in Sasebo, Nagasaki, Japan, on 8 October 2017.The 4,626 participants all dressed in a red and white bobble hat, red and white striped shirt, blue jeans and glasses.
For a complete list of 2017 records, please visit 2017 Guinness Records in Japan.