How many odor can human smell?

Compared to many animals, man’s ability to smell is not very good at all. In fact, it is probable that as men developed, his sense of smell became less and less keen, until today he is chiefly a “visual animal”.
The dog, to take an example at the other extreme, is almost completely an “olfactory animal” meaning he lives by his sense of smell. Here are some comparisons that will show how inferior man is in this respect. In our nose, the area devoted to smelling is actually about the size of a fingernail on each side. In a dog, this area, if the membrane were spread out, would cover more than half the animal’s skin!
In the human brain, where the sensations of smell “register”, about a twentieth of the brain is concerned with smell. One-third of a dog’s brain is concerned with the sense of smell! Man, of course, has developed other senses, organs, and powers that more than make up for this weakness in smelling.

Dog's smell capacity is much higher than Human

In our nose, smells are picked up by delicate hairs that are in the olfactory membrane. But the tips of these “antennae” don’t stick out in the air; they are embedded in a special layer that covers the membrane. This membrane is always moist. If it becomes dry, we no longer can smell! In addition, in ordinary breathing, the stream of air does not pass over this membrane – when we want to smell something.
Before we can smell anything, that substance has to be dissolved in the fatty layer that covers the delicate hairs that pick up the smell. So these substances have to be volatile oils (as those from flowers), or be carried by such oils (as from coffee).
There are five important types such substance that our sense of smell can detect. One is flowery (violet, rose etc.) The second is spicy (lemon, apple etc.). The third is burnt (coffee, tobacco). The fourth is putrescent (cheese, rotten eggs). And the fifth is ethereal (alcohol, camphor etc.)