The type of number that tells you how many atoms of an element are in a single molecule is called a subscript.

Here's an example:

* H₂O (water)

* The subscript "2" after the "H" tells us that there are two hydrogen atoms in a water molecule.

* The subscript "1" after the "O" (even though it's not explicitly written) tells us that there is one oxygen atom in a water molecule.