According to Official Guinness Records,
The largest gathering of people dressed as Dr Seuss characters is 686, achieved by Milford Elementary School (USA) in Milford, New Hampshire, USA, on 22 February 2018.The students and faculty of Milford Elementary School - Heron Pond broke this record as part of their annual Read Across America week.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.