According to Official Guinness Records,
The largest display of international maritime flags is 1,840 and was achieved by Costa Crociere S.p.A. (Italy) in Genoa, Italy, on 5 July 2018. The flags were arranged on a single thread, called "Pavese" in marine terminology, that was 2,150 meters long. The attempt was part of an event to celebrate the 70th anniversary of Costa Crociere cruises.
For a complete list of 2018 records, please visit 2018 Guinness Records in Italy.