According to Official Guinness Records,
The most signatures on a whiteboard is 1,119, and was achieved by Houston Tamil Studies Chair Inc BKM Metroplex Tamil Sangam FETNA and Traditional India (both USA) in Dallas, Texas, USA, on 30 June 2018.The signatures were all written in Tamil.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.