According to Official Guinness Records,
The most signatures on a piece of sports memorabilia is 895, achieved by Ozinga (USA) at Wrigley Field in Chicago, Illinois, USA, on 22 September 2018.This record was attempted in celebration of Ozinga's 90th anniversary and a large Cubs jersey was signed by employees of the company and their families, who were in attendance at the event.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.