Four. Once you correct for the near-complete collapse of mass-market distribution in the US, SF is only slightly down. This may be no consolation to readers or writers; but a greater number of mass-market titles used to have wider distribution, in lots of small outlets frequented by people who don't go to book stores. Those books aren't in those places anymore, and those sales have been lost.
(The equivalent problem in the UK seems to be derived from the collapse of the net pricing scheme, and the consequent rise of mega-discounting by supermarket chains and an ever greater emphasis on fewer and fewer would-be bestsellers.)
There are a few mass-market winners each year, but even they aren't as big winners as they were a decade ago. It's possible that the mass-market format itself is in a death spiral, or just settling down to a new, lower, level of sales after the recent increase in sales of hardcovers and trade paperbacks.