Mahesh Kapasi (b Oct 28, 1948) of Delhi, who has 14,749 letters published in 382 newspapers and magazines since 1974, holds these records as of June 25, 2015:
Most in a month: 115 letters in May 2015.
Most in a single newspaper in a year: 173 letters in Central Chronicle from Raipur, Chhattisgarh between May 1, 2014 and April 30, 2015. 
Most in a single daily:
766 letters in The Pioneer from Delhi since August 1, 1993.Most prizes: 200 Best Letter to the Editor prizes.
Longest letter: A 960-worded letter in Indian Nation on Dec 3, 1996.