# What part of the chemical formula indicates the number of molecules or atoms involved?

For example, in $C {O}_{2}$, the 2 means there are 2 Oxygen atoms ($O$) in one molecule of the compound.
If a formula says ${\left(N {H}_{4}\right)}_{2} S {O}_{4}$ or has similar parentheses, the subscript outside of the parentheses is how many of that part is in the compound (in this case - 2 $N {H}_{4}$ ions).