Subscripts in a molecular formula represent the number of atoms of an element in a molecule?

That's right. When you have a chemical formula, such as SO4, the formula for Sulfate, the subscript means that there are four Oxygen atoms for every one Sulfur atom. Also, when you have really big molecules that are made up of several smaller molecules in them, such as Iron Sulfate, the subscript goes just outside of parenthesis surrounding the smaller molecule's own formula, like this: Fe2(SO4)3