LA Times:

When it comes to the senses, smell often gets short shrift. Scientific research has shown humans can distinguish between 2.3 million and 7.5 million colors. It has also shown we can hear 340,000 different tones. But no one had taken the time to learn how many different smells we can access until now.
The study, however, did not test and find 1 trillion smells but instead extrapolated that astounding figure from a very small sample. The researchers are even bold enough to suggest that 1 trillion is a conservative estimate; they suspect we can distinguish much, much more than that. But the finding brings another and more interesting problem to mind: How much of this information gathered by the nose gets processed and used? What we know is that very little of it reaches the level of our consciousness. What we don't know is the extent to which such a wide variety of scents would meaningfully inform and even guide processes below our self-awareness.

My guess: This sense is like some old country manor that's huge for sure and was once the talk of the town but is now slowly decaying, slowly sinking, and growing more and more vacant.