Question

How many atoms of oxygen (O) are in one formula unit of Mg(NO3)2?

Answer 1

We may use the molecular formula of the compound to determine the number of oxygen atoms in one formula unit. The formula unit is:
Mg(NO₃)₂
Here, we can see that there are two nitrate ions in each mole, and each mole of nitrate ion contains three oxygen atoms. Thus, there are 6 oxygen atoms in each formula unit of magnesium nitrate.

Answer 2

The molecular formula indicates the number of atoms of each element in the molecule.
Oxygen exists as molecules (diatomic molecules). The fundamental particles that make up a sample of Oxygen are pairs of atoms, rather than individual atoms. 
You can count the number of atoms of each element in a chemical formula by multiplying the coefficient by the subscript, recognizing that those coefficients and subscripts that are not shown are equal to 1. So, in our case there are 6 atoms of oxygen.