I don't know the record, but the most I've ever had in a row was 18 (my first 18 games on the 1v1 ladder):
https://www.warlight.net/LadderGames?ID=0&&LadderTeamID=4100&Offset=139 - scroll to the bottom
These were all legit losses and also a pretty good explanation of why I left the 1v1 ladder- and strategy games altogether- to play diplos exclusively for a year.
Edited 7/16/2015 19:12:59