According to Official Guinness Records,
The most stamps in a series is 1,606, entitled Chrysanthemums of the World, and was achieved by Kaifeng Branch of China Post Group Corporation (China) in Kaifeng, Henan, China, on 18 October 2016. The stamps which form the Chrysanthemums of the World series were released from 2012 to 2016 during the annual Chrysanthemum Festival in Kaifeng City.
For a complete list of 2016 records, please visit 2016 Guinness Records in China.