What is the number in a chemical formula that tells the number of specific atoms in the molecule?

The number in a chemical formula that tells the number of specific atoms in the molecules is called SUBSCRIPT. 
For instance, for this molecule: CO2; the subscript is 2 and it tells us exactly the number of atoms of oxygen that is present in carbon dioxide. Subscripts are used in chemical formula to indicate the number of atoms present, it can also be used to indicate the structure of a chemical specie.