A few I remember...
Vic Chesnutt, Black Cat Mainstage, 2003? Around 50 people in a little circle around the stage. Good show, he played "Isadora Duncan" on request.
Lots of sparsely attended Iota shows I can remember over the years, as in around 20-30 people. David Dondero, Lisa Germano, Micah P. Hinson, even Bill Callahan a few years back probably wasn't much more than that.