Re: How many nerves are involved in smelling and what does each nerve smell?

There are approximately 20-50 million olfactory (smell) receptor neurons 
that are situated in the olfactory epithelium of vertebrates. Hairlike 
projections called cilia protrude from these neurons and are on the surface 
of one's nasal cavity, along with mucus. These cilia possess the olfactory 
receptors that actually sense the odorant molecules in the air. In addition 
to these 20-50 million olfactory receptor neurons, there are many, many 
other neurons throughout the brain that are involved in the conscious 
perception of, identification of, and reaction to a smell. 

Regarding your second question, most olfactory receptor neurons sense the 
presence of a set of chemically related odorants and do not appear to be 
specific to only one molecular configuration. For example, an olfactory 
receptor might respond to both propane, which is three carbon atoms linked 
in a chain, and butane, which is four carbon atoms linked in a chain. 
Although a given olfactory receptor may respond to multiple odorants, there 
is usually one odorant to which the receptor responds best. The response of 
an olfactory receptor neuron is thought to be generated by the odorant 
molecule binding to the appropriate kind or kinds of olfactory receptors. 
Olfactory receptors are simply proteins that have a shape that changes to 
start a chemical reaction when certain types of molecules or parts of 
molecules interact with them. This interaction is called binding, and when 
an odorant molecule binds to an olfactory receptor, it eventually produces 
an electrical current in olfactory receptor neurons that makes it more 
likely that the neuron will generate an action potential. An action 
potential is a rapid change in the voltage of the cell, and this rapid 
voltage change is transmitted along the axon of the cell to its axon 
terminals, where the signalling neuron connects with the receiving neuron 
or neurons. 

Finally, I can only partially answer your last question. There are two 
groups of nerve cells that line your nasal cavity. One group is the 
olfactory receptor neurons that I just described and the axons of these 
neurons form the olfactory nerve. They are the ones that can be damaged by 
viral infections, which is the most common way that people lose the ability 
to smell (a condition termed anosmia). Another group of nerve cells respond 
to chemical irritants such as ammonia or high concentrations of normal 
odorants. The axons from these nerve cells form part of what is called the 
trigeminal nerve. I think that the remaining smells are, as you've already 
discovered,  noxious stimulation of these nerve cells, but there are 
probably different molecules that stimulate the cells for each odor. The 
signals from these neurons are meant to protect you from potentially 
dangerous stimuli, but I've read that many different noxious odorants can 
activate these cells.