According to Official Guinness Records,
The most combine harvesters working simultaneously on a single field is 303, and was achieved by Harvest for Kids (Canada) in Winkler, Manitoba, Canada, on 4 August 2018.The attempt took place along a 300 acre field of wheat, with combine harvesters of many decades facing each other, all working together for 5 full minutes.
For a complete list of 2018 records, please visit 2018 Guinness Records in Canada.