MAD magazine debuted as a comic book, the first issue of which went on sale on July 10, 1952. It turned into a magazine as of #24 and the numbering continued until #550, whereupon the numbering started over again with a new #1. The current issue is #36 so there have been 586 regular issues of MAD.
For the purposes of this discussion, we are not counting annuals, specials, paperbacks, hardcovers or any other form even though some of them contained material published nowhere else.
Of all the writers and artists who contributed to those 586 issues, the most prolific was (note the past-tense) the late Al Jaffee who had new material in 509 issues. But with the last issue of MAD, my friend and collaborator Sergio Aragonés tied Al's record and with the current issue, Sergio has taken the lead with work in 510 issues.
Sergio's work first appeared in issue #76 and he has been in every issue since with the exception of #111.
In case anyone's interested, the next most prolific contributor is writer Dick DeBartolo. For a complete list, check out Mike Slaubaugh's website.. Since Sergio will probably be contributing to further issues, it is doubtful that anyone will ever wrest the title away from him.