According to Official Guinness Records,
The largest gathering of people dressed as reindeer is 505, achieved at an event organised by Association of Ogori-city Life-supportment (Japan), at Ogori City Culture Centre in Ogori, Fukuoka, Japan, on 25 December 2016. The organiser is a public organisation which promotes the normalization principle. They organised the record attempt as a part of their event to gather people irrespective of age, gender, and physical conditions to do the same activity. The 505 participants, including including infants, wore unified reindeer costumes and sang two Christmas songs during the five minute attempt at the venue.
For a complete list of 2016 records, please visit 2016 Guinness Records in Japan.