According to Wikipedia (which has some very large gaps in what I am sure is a highly inaccurate record) the 101st (1989-1991) set the record for most special elections with 12.
The 71st Congress (1929-1931) actually has the record, with 27 special elections.
As you and I both noted, wiki is less than comprehensive in it's information on this topic. May I ask where you did manage to find that number? Also 27 special elections seems like an extremely large number. Why were there so many?
, by Micheal J. Dubin.
In addition to the special elections, there were also two vacancies that were not filled by special elections. I can tell you the dates, congressional districts, and even candidates of all the special elections, if you'd like.
I don't know why there were so many vacancies, though I guess it might have something to do with the Great Depression.
