I believe there are 8 in total. These were completed prior to production shutting down when the strike hit. I have not heard if there will be another batch later this season. There were originally going to be 16.
I read an interview with Matthew Fox and he said there will be three more seasons (4, 5 & 6) and each will have 16 episodes. The writers' strike may have forced them to cut short this series, but it seems unlikely -- it's too much of a cash cow.