The sixth, certainly; it was the first that had become routine enough to send a professional geologist along. I'd go so far as to say that most of the scientific value in 12-16 was in making 17 possible. There absolutely should have been a seventh, eighth and probably ninth; scientific targets had already been picked for the next three IIRC.

After that, maybe not so much. But the program was designed with ~nine landings in mind, and that's what we should have done.