± 20 people:
Lali Puna, Prefuse 73, Annie
±10 people:
Jonathan Richman, Ital
These were all in venues with capacity of hundreds.
Saddest gigs were Maher Shalal Bash Haz or Mark Ernestus ft. Tikiman. They both started out to a rather large crowd, but at the end, only a few people remained.
But hey, beter to play for a small audience than to play for a large crowd who pay not attention at all. I once saw Josephine Foster at opening of ATP festival, and it seems as though noone was interested. She even complained several times herself (in a nice, loving way), but as noone was paying attention noone noticed she did.
Also saw Jason Molina with band once, while a large portion of the crowd seemed to be taling amongst each other.