The boats are still running - we just took a boat downriver to HatSa two days ago. We also took boats upriver from Nong Khiaw all the way to HatSa this last week and a half, a spectacular ride that I highly recommend. Right now, the boats run about once a day and depart from Nong Khiaw, Mong Ngoi, Muang Khua and HatSa.
Regarding the minimum number of people, I am not convinced this is really the case. Only one of the four boat rides we took had more than 10 people, but we did not pay an inflated price despite not being full. That said, they did try to squeeze us for more cash, however, when we said we would just wait another day, they relinquished and let us travel at the normal listed price.
Enjoy the ride!! It was one of our most favorite trips we took during the last nine months of traveling.