Which one is correct?
I am talking to air
I am talking to the air
I smell peach
I smell peaches
I smell a peach
What's the difference? air or smell is uncountable right so there should be a the or an a?
As to your other sentences, they all are correct with different meanings:
I smell peach = I sense an aroma which is that of the fruit.
I smell peaches = I smell the aroma of several peaches
I smell a peach = I smell the aroma of one peach.