In "19 Down" Grissom says he was "suspected of killing at least 14 people," but I don't think the first couple was included in that count. Males 2-8 were found before Haskell was caught, but the first one wasn't found until after the seminar (end of "19 Down"). So I believe the proper count is 16, although he may have been tried for only 14 (or maybe 7, since the women were never found).
While Haskell may have had help, he was directly involved in all of the earlier killings (he claimed sole responsibilty for them all). After he was caught and put in jail, one of his accomplices killed one of the others, and three other people (the couple from "Leave out all the Rest" and the man in "19 Down").