To me, the idea that you can smell a trillion smells is somehow baffling. What does that even mean? To talk it through to myself, I wrote a story about it on Medium which somehow partially metastasized into a history of the classification of smells:
In What the Nose Knows, Avery Gilbert describes the history of the people who have tried to force an order onto smells. It begins, somehow predictably, with the godfather of scientific systemization, Linnaeus. The man who had brought us the taxonomic classification for animals, with names such as Felis catus and Caenorhabditis elegans, attempted to do the same for odors. He decided there must be a discrete set of classes of odors, which include fragrant, spicy, musky, garlicky, goaty, foul, and nauseating. This was later refined by Hendrik Zwaardermaker who added the classesethereal and empyreumatic, as well as adding subclasses for each class. Next came Hans Henning, who decided that smells lie on an odor prism. Each vertex was a specific quality of odor — flowery, foul, fruity, spicy, burnt, and resinous — and the distance of a small from each vertex was the relative contribution of that quality to the odor. This gave odors a space and direction and possibly even dynamics from one point to another. But it also didn’t work.
I think the key to understanding the concept of ‘a trillion smells’ is that there are so many basis elements; there are only three primary colors but there are many, many primary odorants.