Answer: The correct answer is for the blank is- number of atoms.
Chemical formula is a respresentation of proportion of different elements that are present in a compound.
When a compound possesses more than one atom of an element, its chemical symbols has the respective number in its subscript.
In other words, subscript in a chemical formula refers to the number of atoms of that element.
For Example- chemical formula of glucose is C6H12O6, which represents 6 number of carbon atoms, 12 number of hydrogen atoms, and 6 number of oxygen atoms in the glucose.