How do I identify the numbers and types of atoms represented by chemical formulas?

Take for example, water,

There are 2 H atoms and 1 O atom.

Rust; has 2 Fe atoms and 3 O atoms.

More examples? :)

I was just giving an exa,ple I'm not really sure I need to know how to know how stuff makes up other stuff, I'm really confuzed

In the chemical formula 2SO2, how many total atoms of oxygen (O) are there?

How many do you think there are? :)