According to Official Guinness Records,
The most words on a postage stamp is 1969, and was achieved by the United Nations Postal Administration (USA) in New York, New York, USA, on 20 October 2017. The record breaking stamp features the entire text of the Universal Declaration of Human Rights and its pre-amble in French. It is available as a set with the same text translated into both English (1771 words) and German (1660 words).
For a complete list of 2017 records, please visit 2017 Guinness Records in United States.